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  • A packet is the unit of data that is routed between an origin and a destination on the Internet or any other packet-switched network. When any file (e-mail message, HTML file, Graphics Interchange Format file, Uniform Resource Locator request, and so forth) is sent from one place to another on the Internet, the Transmission Control Protocol (TCP) layer of TCP/IP divides the file into "chunks" of an efficient size for routing. Each of these packets is separately numbered and includes the Internet address of the destination. The individual packets for a given file may travel different routes through the Internet. When they have all arrived, they are reassembled into the original file (by the TCP layer at the receiving end).
  • A packet-switching scheme is an efficient way to handle transmissions on a connectionless network such as the Internet. An alternative scheme, circuit-switched , is used for networks allocated for voice connections. In circuit-switching, lines in the network are shared among many users as with packet-switching, but each connection requires the dedication of a particular path for the duration of the connection. "Packet" and "datagram" are similar in meaning. A protocol similar to TCP, the User Datagram Protocol(UDP) uses the term datagram.


  • PacketHound is a product that aims to help an enterprise regulate traffic that might otherwise slow services down for all users of a local area network . PacketHound is intended to address the concern of businesses and universities about the downloading of large files, especially music files in the MP3 format, using Napster, Gnutella , and similar approaches. PacketHound can also monitor and reduce the number of streaming media files that are downloaded by network users. Both businesses and universities are concerned not only about such traffic slowing down the network for other uses, but also about the possibility of being sued by music companies for loss of intellectual property. According to Palisade Systems, the developers of PacketHound, Napster and Gnutella traffic has had a significant effect in performance on a number of university networks.
  • Although Gnutella uses a known port number that a network firewall server could screen for, the port number can be changed by a sophisticated user. The makers of PacketHound claim to be the only product that can monitor and block certain traffic based on the characteristics of the request and response flow, although they do not describe their approach in detail. PacketHound customers can also use the product to monitor without blocking and to also permit or block given traffic at different times of the day. PacketHound is not installed in a firewall server but as a separate PC with an Ethernet card and running NetBSD. The machine's presence is said to be transparent to the network. When PacketHound discerns a pattern of traffic that meets the blocking criteria, it returns a reset packet to the requesting machine. The user sees a "Connection reset by host" message."
  • The company also sells PacketPup, a downloadable program that lets a company monitor bandwidth usage on a network. A similar product called PacketShaper (from Packeteer) analyzes and classifies applications in use on the network in terms of their bandwidth and other behavior.


  • Pagejacking is stealing the contents of a Web site by copying some of its pages, putting them on a site that appears to be the legitimate site, and then inviting people to the illegal site by deceptive means - for example, by having the contents indexed by major search engine s whose results in turn link users to the illegal site. By moving enough of a Web site's content as well as the page descriptor information (known as information) within each page, pagejackers can then submit the illegal site to major search engines for indexing. Users of the search engine sites may then receive results from both the illegitimate as well as the legitimate site and can easily be misled to link to the wrong one. Users linking to the illegitimate site may find themselves redirected to a pornographic or other unwanted site. As an additional annoyance, users subjected to pagejacking may also encounter mousetrapping, in which clicking the Back button with the mouse does not lead out of the illegal site but only to the viewing of additional unwanted pages. To escape, the user may need to close the browser or even restart the operating system.
  • Web users who enter Web page addresses (known as URLs) directly on their Web browser address line, by selecting it from a bookmark, or by clicking on a properly coded link on another site will not be subject to pagejacking. The problem most typically occurs when clicking site descriptions that result from searches at major search engine sites. Although the practice was not new at the time, the New York Times on September 23, 1999, carried a page one story about an Australian company that had pagejacked a number of corporate sites, adding pornographic links or ads, and mousetrapping users. Australian officials were reported to be considering civil or criminal charges and a U.S. Federal judge in Virginia, where the original Internet site registration company is located, ordered the sites to lose their Web registrations.


  • In computers, parity (from the Latin paritas: equal or equivalent) refers to a technique of checking whether data has been lost or written over when it's moved from one place in storage to another or when transmitted between computers.
  • Here's how it works: An additional binary digit, the parity bit , is added to a group of bits that are moved together. This bit is used only for the purpose of identifying whether the bits being moved arrived successfully. Before the bits are sent, they are counted and if the total number of data bits is even, the parity bit is set to one so that the total number of bits transmitted will form an odd number. If the total number of data bits is already an odd number, the parity bit remains or is set to 0. At the receiving end, each group of incoming bits is checked to see if the group totals to an odd number. If the total is even, a transmission error has occurred and either the transmission is retried or the system halts and an error message is sent to the user.
  • The description above describes how parity checking works within a computer. Specifically, the Peripheral Component Interconnect bus and the I/O bus controller use the odd parity method of error checking. Parity bit checking is not an infallible error-checking method since it's possible that two bits could be in error in a transmission, offsetting each other. For transmissions within a personal computer, this possibility is considered extremely remote. In some large computer systems where data integrity is seen as extremely important, three bits are allocated for parity checking.
  • Parity checking is also used in communication between modems. Here, parity checking can be selected to be even (a successful transmission will form an even number) or odd. Users may also select no parity , meaning that the modems will not transmit or check a parity bit. When no parity is selected (or defaulted), it's assumed that there are other forms of checking that will detect any errors in transmission. No parity also usually means that the parity bit can be used for data, speeding up transmission. In modem-to-modem communication, the type of parity is coordinated by the sending and receiving modems before the transmission takes place.

Parse /Parser:

  • To parse is to analyze something in an orderly way. In linguistics, to parse is to divide words and phrases into different parts in order to understand relationships and meaning. For example, English students are sometimes asked to parse a sentence by dividing it into subject and predicate, and then into dependent phrases, modifiers, and so forth. In general, to parse someone's writing or speech simply means to interpret it.
  • In computers, to parse is to divide a computer language statement into parts that can be made useful for the computer. A parser in a program compiler is a program that takes each program statement that a developer has written and divides it into parts (for example, the main command, options, target objects, their attributes, and so forth) that can then be used for developing further actions or for creating the instructions that form an executable program.
  • In computer technology, a parser is a program, usually part of a compiler , that receives input in the form of sequential source program instructions, interactive online commands, markup tags, or some other defined interface and breaks them up into parts (for example, the nouns (objects), verbs (methods), and their attributes or options) that can then be managed by other programming (for example, other components in a compiler). A parser may also check to see that all input has been provided that is necessary.


  • In personal computers, a partition is a logical division of a hard disk created so that you can have different operating system s on the same hard disk or to create the appearance of having separate hard drives for file management, multiple users, or other purposes. A partition is created when you format the hard disk. Typically, a one-partition hard disk is labelled the "C:" drive ("A:" and "B:" are typically reserved for diskette drives). A two-partition hard drive would typically contain "C:" and "D:" drives. (CD-ROM drives typically are assigned the last letter in whatever sequence of letters have been used as a result of hard disk formatting, or typically with a two-partition, the "E:" drive.)
  • When you boot an operating system into your computer, a critical part of the process is to give control to the first sector on your hard disk. It includes a partition table that defines how many partitions the hard disk is formatted into, the size of each, and the address where each partition begins. This sector also contains a program that reads in the boot sector for the operating system and gives it control so that the rest of the operating system can be loaded into random access memory.
  • Boot virus es can put the wrong information in the partition sector so that your operating system can't be located. For this reason, you should have a back-up version of your partition sector on a diskette known as a bootable floppy.

Passive FTP:

  • Passive FTP (sometimes referred to as PASV FTP because it involves the FTP PASV command) is a more secure form of data transfer in which the flow of data is set up and initiated by the File Transfer Program (FTP) client rather than by the FTP server program. Separate FTP client programs, such as WS_FTP Pro, usually allow the user to select passive FTP. Most Web browsers (which act as FTP clients) use passive FTP by default because corporations prefer it as a safety measure. As a general rule, any coprorate firewall server, which exists in order to protect an internal network from the outside world, recognizes input from the outside only in response to user requests that were sent out requesting the input. The use of passive FTP ensures all data flow initiation comes from inside the network rather than from the outside.
  • How It Works:
  • Using normal or passive FTP, a client begins a session by sending a request to communicate through TCP port 21, the port that is conventionally assigned for this use at the FTP server. This communication is known as the Control Channel connection.Using "normal" FTP communication, the client requestor also includes in the same PORT command packet on the Control Channel a second port number that is to be used when data is to be exchanged; the port-to-port exchange for data is known as the Data Channel. The FTP server then initiates the exchange from its own port 20 to whatever port was designated by the client. However, because the server-initiated communication is no longer controlled by the client and can't be correlated by a firewall to the initial request, the potential exists for uninvited data to arrive from anywhere posing as a normal FTP transfer.
  • Using passive FTP, a PASV command is sent instead of a PORT command. Instead of specifying a port that the server can send to, the PASV command asks the server to specify a port it wishes to use for the Data Channel connection. The server replies on the Control Channel with the port number which the client then uses to initiate an exchange on the Data Channel. The server will thus always be responding to client-initiated requests on the Data Channel and the firewall can coorelate these.

PDC - Primary Domain Controller:

  • See Primary Domain Controller

PCM - Pulse Code Modulation:

  • See Pulse Code Modulation.


  • 1) Peer-to-peer is a communications model in which each party has the same capabilities and either party can initiate a communication session. Other models with which it might be contrasted include the client/server model and the master/slave model. In some cases, peer-to-peer communications is implemented by giving each communication node both server and client capabilities. In recent usage, peer-to-peer has come to describe applications in which users can use the Internet to exchange files with each other directly or through a mediating server.
  • IBM's Advanced Peer-to-Peer Networking (APPN) is an example of a product that supports the peer-to-peer communication model.
  • 2) On the Internet, peer-to-peer (referred to as P2P) is a type of transient Internet network that allows a group of computer users with the same networking program to connect with each other and directly access files from one another's hard drives. Napster and Gnutella are examples of this kind of peer-to-peer software. Corporations are looking at the advantages of using P2P as a way for employees to share files without the expense involved in maintaining a centralized server and as a way for businesses to exchange information with each other directly.
  • How Does Internet P2P Work?
  • The user must first download and execute a peer-to-peer networking program. (Gnutellanet is currently one of the most popular of these decentralized P2P programs because it allows users to exchange all types of files.) After launching the program, the user enters the IP address of another computer belonging to the network. (Typically, the Web page where the user got the download will list several IP addresses as places to begin). Once the computer finds another network member on-line, it will connect to that user's connection (who has gotten their IP address from another user's connection and so on). Users can choose how many member connections to seek at one time and determine which files they wish to share or password protect.

Perl - Practical Extraction and Reporting Language:

  • Perl (Practical Extraction and Reporting Language) is a script programming language that is similar in syntax to the C language and that includes a number of popular UNIX facilities such as SED, awk, and tr. Perl is an interpreted language that can optionally be compiled just before execution into either C code or cross-platform bytecode . When compiled, a Perl program is almost (but not quite) as fast as a fully precompiled C language program. Perl is regarded as a good choice for developing common gateway interface (CGI ) programs because it has good text manipulation facilities (although it also handles binary files). It was invented by Larry Wall.
  • In general, Perl is easier to learn and faster to code in than the more structured C and C++ languages. Perl programs can, however, be quite sophisticated. Perl tends to have devoted adherents.
  • plug-ins can be installed for some servers (Apache , for example) so that Perl is loaded permanently in memory, thus reducing compile time and resulting in faster execution of CGI Perl scripts.

PGP - Pretty Good Privacy:

  • See Pretty Good Privacy.

Photonic Switching:

  • See Lambda Switching.

PHP - Personal Home Page:

  • In Web programming, PHP is a script language and interpreter that is freely available and used primarily on Linux Web servers. PHP, originally derived from Personal Home Page Tools, now stands for PHP: Hypertext Preprocessor, which the PHP FAQ describes as a "recursive acronym."
  • PHP is an alternative to Microsoft's Active Server Page (ASP) technology. As with ASP, the PHP script is embedded within a Web page along with its HTML . Before the page is sent to a user that has requested it, the Web server calls PHP to interpret and perform the operations called for in the PHP script.
  • An HTML page that includes a PHP script is typically given a file name suffix of ".php" ".php3," or ".phtml". Like ASP, PHP can be thought of as "dynamic HTML pages," since content will vary based on the results of interpreting the script. PHP is free and offered under an open source license.


  • To find out the dot address (such as for a given domain name, Windows users can go to their MS DOS prompt screen and enter: ping xxx.yyy where xxx is the second-level domain name like "whatis" and yyy is the top-level domain name like "com").
  • Ping is a basic Internet program that lets you verify that a particular IP address exists and can accept requests. The verb ping means the act of using the ping utility or command. Ping is used diagnostically to ensure that a host computer you are trying to reach is actually operating. If, for example, a user can't ping a host, then the user will be unable to use the File Transfer Protocol (FTP ) to send files to that host. Ping can also be used with a host that is operating to see how long it takes to get a response back. Using ping, you can learn the number form of the IP address from the symbolic domain name (see "Tip").
  • Loosely, ping means "to get the attention of" or "to check for the presence of" another party online. Ping operates by sending a packet to a designated address and waiting for a response. The computer acronym (for Packet Internet or Inter-Network Groper) was contrived to match the submariners' term for the sound of a returned sonar pulse.
  • Ping can also refer to the process of sending a message to all the members of a mailing list requesting an ACK (acknowledgement code). This is done before sending e-mail in order to confirm that all of the addresses are reachable.

PKI - Public Key Infrastructure:

  • See Public Key Infrastructure.


Polish Notation:

  • Polish notation, also known as prefix notation, is a symbolic logic invented by Polish mathematician Jan Lukasiewicz in the 1920's. When using Polish notation, the instruction (operation) precedes the data (operands). In Polish notation, the order (and only the order) of operations and operands determines the result, making parentheses unnecessary.
  • <a> <img></a> The notation for the expression 3(4 +5) could be expressed as
  • x 3 + 4 5
  • This contrasts with the traditional algebraic methodology for performing mathematical operations, the Order of Operations. (The mnemonic device for remembering the Order of Operations is "Please Excuse My Dear Aunt Sally" - parentheses, exponents, multiplication, division, addition, subtraction). In the expression 3(4+5), you would work inside the parentheses first to add four plus five and then multiply the result by three.
  • In the early days of the calculator , the end-user had to write down the results of their intermediate steps when using the algebraic Order of Operations. Not only did this slow things down, it provided an opportunity for the end-user to make errors and sometimes defeated the purpose of using a calculating machine. In the 1960's, engineers at Hewlett-Packard decided that it would be easier for end-users to learn Jan Lukasiewicz' logic system than to try and use the Order of Operations on a calculator. They modified Jan Lukasiewicz's system for a calculator keyboard by placing the instructions (operators) after the data. In homage to Jan Lukasiewicz' Polish logic system, the engineers at Hewlett-Packard called their modification reverse Polish notation (RPN).
  • The notation for the expression 3(4+5) would now be expressed as
  • 4 5 + 3 x
  • or it could be further simplified to
  • 3 4 5 + x
  • Reverse Polish notation provided a straightforward solution for calculator or computer software mathematics because it treats the instructions (operators) and the data (operands) as "objects" and processes them in a last-in, first-out (LIFO) basis. This is called a "stack method". (Think of a stack of plates. The last plate you put on the stack will be the first plate taken off the stack.)
  • Modern calculators with memory functions are sophisticated enough to accommodate the use of the traditional algebraic Order of Operations, but users of RPN calculators like the logic's simplicity and continue to make it profitable for Hewlett-Packard to manufacture RPN calculators. Some of Hewlett Packard's latest calculators are capable of both RPN and algebraic logic.

POP Point-of-Presence:

  • A Point-of-Presence (POP) is an access point to the Internet. A POP necessarily has a unique Internet Protocol (IP) address. Your Internet service provider (ISP ) or online service provider (such as AOL) has a point-of-presence on the Internet and probably more than one. The number of POPs that an ISP or OSP has is sometimes used as a measure of its size or growth rate.
  • A POP may actually reside in rented space owned by the telecommunications carrier (such as Sprint) to which the ISP is connected. A POP usually includes routers, digital/analog call aggregators, servers, and frequently frame relays or ATM switches.

POP3 - Post Office Protocol 3:

  • Post Office Protocol 3 (POP3) is the most recent version of a standard protocol for receiving e-mail. POP3 is a client/server protocol in which e-mail is received and held for you by your Internet server. Periodically, you (or your client e-mail receiver) check your mail-box on the server and download any mail. POP3 is built into the Netmanage suite of Internet products and one of the most popular e-mail products, Eudora. It's also built into the Netscape and Microsoft Internet Explorer browsers.
  • An alternative protocol is Internet Message Access Protocol (IMAP ). With IMAP, you view your e-mail at the server as though it was on your client computer. An e-mail message deleted locally is still on the server. E-mail can be kept on and searched at the server.
  • POP can be thought of as a "store-and-forward" service. IMAP can be thought of as a remote file server. POP and IMAP deal with the receiving of e-mail and are not to be confused with the Simple Mail Transfer Protocol (SMTP ), a protocol for transferring e-mail across the Internet. You send e-mail with SMTP and a mail handler receives it on your recipient's behalf. Then the mail is read using POP or IMAP. The conventional port number for POP3 is 110

Port 80:

  • On a Web server or Hypertext Transfer Protocol daemon , port 80 is the port that the server "listens to" or expects to receive from a Web client, assuming that the default was taken when the server was configured or set up. A port can be specified in the range from 0-65536 on the NCSA server. However, the server administrator configures the server so that only one port number can be recognized. By default, the port number for a Web server is 80. Experimental services may sometimes be run at port 8080.

Port 110:

  • See POP3

Port Forwrding:

  • Port forwarding, or tunneling, is a way to forward insecure TCP (not UDP) traffic through SSH Secure Shell. For example, you can secure POP3, SMTP, and HTTP connections that would otherwise be insecure. There are two kinds of port forwarding: local and remote forwarding. They are also called outgoing and incoming tunnels, respectively.

Port Mirroring:

  • Port mirroring, also known as a roving analysis port, is a method of monitoring network traffic that forwards a copy of each incoming and outgoing packet from one port of a network switch to another port where the packet can be studied. A network administrator uses port mirroring as a diagnostic tool or debugging feature, especially when fending off an attack. It enables the administrator to keep close track of switch performance and alter it if necessary. Port mirroring can be managed locally or remotely.
  • An administrator configures port mirroring by assigning a port from which to copy all packets and another port where those packets will be sent. A packet bound for or heading away from the first port will be forwarded onto the second port as well. The administrator places a protocol analyzer on the port receiving the mirrored data to monitor each segment separately. The analyzer captures and evaluates the data without affecting the client on the original port.
  • The monitor port may be a port on the same SwitchModule with an attached RMON probe, a port on a different SwitchModule in the same hub, or the SwitchModule processor. Port mirroring can consume significant CPU resources while active. Better choices for long-term monitoring may include a passive tap like an optical probe or an Ethernet repeater.

Port Number:

  • A Port Number is a way to identify a specific process to which an Internet or other network message is to be forwarded when it arrives at a server. For the Transmission Control Protocol and the User Datagram Protocol, a port number is a 16-bit integer that is put in the header appended to a message unit. This port number is passed logically between client and server transport layers and physically between the transport layer and the Internet Protocol layer and forwarded on.
  • For example, a request from a client (perhaps on behalf of you at your PC) to a server on the Internet may request a file be served from that host's File Transfer Protocol (FTP) server or process. In order to pass your request to the FTP process in the remote server, the Transmission Control Protocol (TCP ) software layer in your computer identifies the port number of 21 (which by convention is associated with an FTP request) in the 16-bit port number integer that is appended to your request. At the server, the TCP layer will read the port number of 21 and forward your request to the FTP program at the server.
  • Some services or processes have conventionally assigned permanent port numbers. These are known as well-known port numbers . In other cases, a port number is assigned temporarily (for the duration of the request and its completion) from a range of assigned port numbers. This is called an ephemeral port number.

POSIX - Portable Operating System Interface:

  • POSIX (Portable Operating System Interface) is a set of standard operating system interfaces based on the UNIX operating system. The need for standardization arose because enterprises using computers wanted to be able to develop programs that could be moved among different manufacturer's computer systems without having to be recoded. UNIX was selected as the basis for a standard system interface partly because it was "manufacturer-neutral." However, several major versions of UNIX existed so there was a need to develop a common denominator system.
  • Informally, each standard in the POSIX set is defined by a decimal following the POSIX. Thus, POSIX.1 is the standard for an application program interface in the C language. POSIX.2 is the standard shell and utility interface (that is to say, the user's command interface with the operating system). These are the main two interfaces, but additional interfaces, such as POSIX.4 for thread management, have been developed or are being developed. The POSIX interfaces were developed under the auspices of the Institute of Electrical and Electronics Engineers (IEEE).
  • POSIX.1 and POSIX.2 interfaces are included in a somewhat larger interface known as the X/Open Programming Guide 4.2 (also known as the "Single UNIX Specification" and "UNIX 95"). The Open Group , an industry standards group, owns the UNIX trademark and can thus "brand" operating systems that conform to the interface as "UNIX" systems. IBM's OS/390 is an example of an operating system that includes a branded UNIX interface.

POTS - Plain Old Telephone Service:

  • POTS is a term sometimes used in discussion of new telephone technologies in which the question of whether and how existing voice transmission for ordinary phone communication can be accommodated. For example, Asymmetric Digital Subscriber Line and Integrated Services Digital Network connections provide some part of their channels for "plain old telephone service" while providing most of their bandwidth for digital data transmission.

PPP - Point-to-Point Protocol:

  • Point-to-Point Protocol (PPP) is a protocol for communication between two computers using a serial interface, typically a personal computer connected by phone line to a server. For example, your Internet server provider may provide you with a PPP connection so that the provider's server can respond to your requests, pass them on to the Internet, and forward your requested Internet responses back to you. PPP uses the Internet protocol (IP ) (and is designed to handle others). It is sometimes considered a member of the TCP/IP suite of protocols. Relative to the Open Systems Interconnection (OSI) reference model, PPP provides layer 2 (data-link layer) service. Essentially, it packages your computer's TCP/IP packets and forwards them to the server where they can actually be put on the Internet.
  • PPP is a full-duplex protocol that can be used on various physical media, including twisted pair or fiber optic lines or satellite transmission. It uses a variation of High Speed Data Link Control (HDLC) for packet encapsulation. PPP is usually preferred over the earlier de facto standard Serial Line Internet Protocol (SLIP) because it can handle synchronous as well as asynchronous communication. PPP can share a line with other users and it has error detection that SLIP lacks. Where a choice is possible, PPP is preferred.

PPTP - Point-to-Point Tunneling Protocol:

  • Point-to-Point Tunneling Protocol (PPTP) is a protocol (set of communication rules) that allows corporations to extend their own corporate network through private "tunnels" over the public Internet. Effectively, a corporation uses a wide-area network as a single large local area network. A company no longer needs to lease its own lines for wide-area communication but can securely use the public networks. This kind of interconnection is known as a virtual private network (VPN).
  • PPTP, a proposed standard sponsored by Microsoft and other companies, and Layer 2 Tunneling Protocol, proposed by Cisco Systems, are among the most likely proposals as the basis for a new Internet Engineering Task Force (IETF) standard. With PPTP, which is an extension of the Internet's Point-to-Point Protocol (PPP), any user of a PC with PPP client support is able to use an independent service provider (ISP) to connect securely to a server elsewhere in the user's company. Also see VPN.

Pretty Good Privacy - PGP:

  • Pretty Good Privacy (PGP) is a popular program used to encrypt and decrypt e-mail over the Internet. It can also be used to send an encrypted digital signature that lets the receiver verify the sender's identity and know that the message was not changed en route. Available both as freeware and in a low-cost commercial version, PGP is the most widely used privacy-ensuring program by individuals and is also used by many corporations. Developed by Philip R. Zimmermann in 1991, PGP has become a de facto standard for e-mail security. PGP can also be used to encrypt files being stored so that they are unreadable by other users or intruders.
  • How It Works
  • PGP uses a variation of the public key system. In this system, each user has a publicly known encryption key and a private key known only to that user. You encrypt a message you send to someone else using their public key. When they receive it, they decrypt it using their private key. Since encrypting an entire message can be time-consuming, PGP uses a faster encryption algorithm to encrypt the message and then uses the public key to encrypt the shorter key that was used to encrypt the entire message. Both the encrypted message and the short key are sent to the receiver who first uses the receiver's private key to decrypt the short key and then uses that key to decrypt the message.
  • PGP comes in two public key versions - Rivest-Shamir-Adleman (RSA) and Diffie-Hellman. The RSA version, for which PGP must pay a license fee to RSA, uses the IDEA algorithm to generate a short key for the entire message and RSA to encrypt the short key. The Diffie-Hellman version uses the CAST algorithm for the short key to encrypt the message and the Diffie-Hellman algorithm to encrypt the short key.
  • For sending digital signatures, PGP uses an efficient algorithm that generates a hash (or mathematical summary) from the user's name and other signature information. This hash code is then encrypted with the sender's private key. The receiver uses the sender's public key to decrypt the hash code. If it matches the hash code sent as the digital signature for the message, then the receiver is sure that the message has arrived securely from the stated sender. PGP's RSA version uses the MD5 algorithm to generate the hash code. PGP's Diffie-Hellman version uses the SHA-1 algorithm to generate the hash code.
  • To use PGP, you download or purchase it and install it on your computer system. Typically, it contains a user interface that works with your customary e-mail program. You also need to register the public key that your PGP program gives you with a PGP public-key server so that people you exchange messages with will be able to find your public key. Network Associates maintains an LDAP/HTTP public key server that has 300,000 registered public keys. This server has mirror sites around the world.
  • Where Can You Use PGP?
  • Originally, the U.S. government restricted the exportation of PGP technology. Today, however, PGP encrypted e-mail can be exchanged with users outside the U.S if you have the correct versions of PGP at both ends. Unlike most other encryption products, the international version is just as secure as the domestic version.
  • The freely available PGP cannot legally be used for commercial purposes - for that, one must obtain the commercial version from Network Associates (formerly PGP, Inc.). There are several versions of PGP in use. Add-ons can be purchased that allow backwards compatibility for newer RSA versions with older versions. However, the Diffie-Hellman and RSA versions of PGP do not work with each other since they use different algorithms.

Pre-Boot Execution Environment:

  • The Preboot Execution Environment (PXE) is an industry standard client/server interface that allows networked computers that are not yet loaded with an operating system to be configured and booted remotely by an administrator. The PXE code is typically delivered with a new computer on a read-only memory chip or boot disk that allows the computer (a client) to communicate with the network server so that the client machine can be remotely configured and its operating system can be remotely booted. PXE provides three things:
  • 1) The Dynamic Host Configuration Protocol (DHCP), which allows the client to receive an IP address to gain access to the network servers.
  • 2) A set of application program interfaces (API) that are used by the client's Basic Input/Output Operating System (BIOS ) or a downloaded Network Bootstrap Program (NBP) that automates the booting of the operating system and other configuration steps.
  • 3) A standard method of initializing the PXE code in the PXE ROM chip or boot disk.
  • The PXE process consists of the client notifying the server that it uses PXE. If the server uses PXE, it sends the client a list of boot servers that contain the operating systems available. The client finds the boot server it needs and receives the name of the file to download. The client then downloads the file using Trivial File Transfer Protocol (Trivia File Transfer Protocol ) and executes it, which loads the operating system. If a client is equipped with PXE and the server is not, the server ignores the PXE code preventing disruption in the DHCP and Bootstrap Protocol (BP) operations.
  • The advantages of using PXE include:
  • * The client machine does not necessarily need an operating system or even a hard disk.
  • * The client machine can be rebooted in the event of hardware or software failure. This allows the administrator to diagnose and perhaps fix the problem.
  • * Since PXE is vendor-independent, new types of computers can easily be added to the network.

Primary Domain Controler - PDC

  • Primary Domain Controller (PDC) and backup domain controller (BDC) are roles that can be assigned to a server in a network of computers that use the Windows NT operating system. Windows NT uses the idea of a domain to manage access to a set of network resources (applications, printers, and so forth) for a group of users. The user need only to log in to the domain to gain access to the resources, which may be located on a number of different servers in the network. One server, known as the primary domain controller, manages the master user database for the domain. One or more other servers are designated as backup domain controllers. The primary domain controller periodically sends copies of the database to the backup domain controllers. A backup domain controller can step in as primary domain controller if the PDC server fails and can also help balance the workload if the network is busy enough.
  • In Windows NT, a domain combines some of the advantages of a workgroup (a group of users who exchange access to each others' resources on different computers) and a directory (a group of users who are managed centrally by an administrator). The domain concept not only allows a user to have access to resources that may be on different servers, but it also allows one domain to be given access to another domain in a trust relationship. In this arrangement, the user need only log in to the first domain to also have access to the second domain's resources as well.
  • In a Windows NT network, not all servers need to be a PDC or BDC. A server can be designated as a member server whose resources become part of a domain without having a role in the logon process. Setting up and maintaining PDCs and BDCs and domain information is a major activity for the administrator of a Windows NT network. In Windows 2000, the domain controller concept is retained but the PDC and BDC server roles are generally replaced by the Active Directory.

Primary Rate Interface:

  • In the Integrated Services Digital Network (ISDN), there are two levels of service: the Basic Rate Interface (BRI), intended for the home and small enterprise, and the Primary Rate Interface (PRI), for larger users. Both rates include a number of B-channels and a D-channel. Each B-channel carries data, voice, and other services. The D-channel carries control and signaling information.
  • The Basic Rate Interface consists of two 64 Kbps B-channels and one 16 Kbps D-channel. Thus, a Basic Rate Interface user can have up to 128 Kbps service. The Primary Rate Interface consists of 23 B-channels and one 64 Kpbs D-channel using a T-1 line or 30 B-channels and 1 D-channel using an E1 line. Thus, a Primary Rate Interface user on a T-1 line can have up to 1.544 Mbps service or up to 2.048 Mbps service on an E1 line. PRI uses the Q.931 protocol over the D-channel.
  • The Primary Rate Interface channels are carried on a T-carrier system line (in the U.S., Canada, and Japan) or an E-carrier line (in other countries) and are typically used by medium to large enterprises. The 23 (or 30) B-channels can be used flexibly and reassigned when necessary to meet special needs such as videoconferences. The Primary Rate user is hooked up directly to the telephone company central office. For more information, see ISDN.

Proxy Server:

  • In an enterprise that uses the Internet, a proxy server is a server that acts as an intermediary between a workstation user and the Internet so that the enterprise can ensure security, administrative control, and caching service. A proxy server is associated with or part of a gateway server that separates the enterprise network from the outside network and a firewall server that protects the enterprise network from outside intrusion.
  • A proxy server receives a request for an Internet service (such as a Web page request) from a user. If it passes filtering requirements, the proxy server, assuming it is also a cache server, looks in its local cache of previously downloaded Web pages. If it finds the page, it returns it to the user without needing to forward the request to the Internet. If the page is not in the cache, the proxy server, acting as a client on behalf of the user, uses one of its own IP addresses to request the page from the server out on the Internet. When the page is returned, the proxy server relates it to the original request and forwards it on to the user.
  • To the user, the proxy server is invisible; all Internet requests and returned responses appear to be directly with the addressed Internet server. (The proxy is not quite invisible; its IP address has to be specified as a configuration option to the browser or other protocol program.)
  • An advantage of a proxy server is that its cache can serve all users. If one or more Internet sites are frequently requested, these are likely to be in the proxy's cache, which will improve user response time. In fact, there are special servers called cache servers. A proxy can also do logging.
  • The functions of proxy, firewall, and caching can be in separate server programs or combined in a single package. Different server programs can be in different computers. For example, a proxy server may in the same machine with a firewall server or it may be on a separate server and forward requests through the firewall.

Private Key:

  • In cryptography, a private or secret key is an encryption/decryption key known only to the party or parties that exchange secret messages. In traditional secret key cryptography, a key would be shared by the communicators so that each could encrypt and decrypt messages. The risk in this system is that if either party loses the key or it is stolen, the system is broken. A more recent alternative is to use a combination of public and private keys. In this system, a public key is used together with a private key. See public key infrastructure (PKI) for more information.

Private Port Numbers:

  • See Dynamic Port Numbers


  • Pseudocode (pronounced SOO-doh-kohd) is a detailed yet readable description of what a computer program or algorithm must do, expressed in a formally-styled natural language rather than in a programming language. Pseudocode is sometimes used as a detailed step in the process of developing a program. It allows designers or lead programmers to express the design in great detail and provides programmers a detailed template for the next step of writing code in a specific programming language.
  • Because pseudocode is detailed yet readable, it can be inspected by the team of designers and programmers as a way to ensure that actual programming is likely to match design specifications. Catching errors at the pseudocode stage is less costly than catching them later in the development process. Once the pseudocode is accepted, it is rewritten using the vocabulary and syntax of a programming language. Pseudocode is sometimes used in conjunction with computer-aided software engineering-based methodologies. It is possible to write programs that will convert a given pseudocode language into a given programming language.

Public Domain Software:

  • Programs that are uncopyrighted because their authors intended to share them with everyone else are in the public domain. The UNIX community has developed a number of such programs over the years. Programs in the public domain can be used without restriction as components of other programs. When reusing such code, it is good to understand its history so that you can be sure it really is in the public domain. Also see shareware, which is programming that is "free" but more or less on a trial basis, and freeware.

Public Key Infrastructure - PKI:

  • A Public Key Infrastructure (PKI) enables users of a basically unsecure public network such as the Internet to securely and privately exchange data and money through the use of a public and a private cryptographic key pair that is obtained and shared through a trusted authority. The public key infrastructure provides for a digital certificate that can identify an individual or an organization and directory services that can store and, when necessary, revoke the certificates. Although the components of a PKI are generally understood, a number of different vendor approaches and services are emerging. Meanwhile, an Internet standard for PKI is being worked on.
  • The public key infrastructure assumes the use of public key cryptography, which is the most common method on the Internet for authenticating a message sender or encrypting a message. Traditional cryptography has usually involved the creation and sharing of a secret key for the encryption and decryption of messages. This secret or private key system has the significant flaw that if the key is discovered or intercepted by someone else, messages can easily be decrypted. For this reason, public key cryptography and the public key infrastructure is the preferred approach on the Internet. (The private key system is sometimes known as symmetric cryptography and the public key system as asymmetric cryptography.)
  • A public key infrastructure consists of:
  • * A certificate authority (CA) that issues and verifies digital certificate. A certificate includes the public key or information about the public key
  • * A registration authority (RA) that acts as the verifier for the certificate authority before a digital certificate is issued to a requestor
  • * One or more directories where the certificates (with their public keys) are held
  • * A certificate management system
  • How Public and Private Key Cryptography Works
  • In public key cryptography, a public and private key are created simultaneously using the same algorithm (a popular one is known as RSA ) by a certificate authority (CA). The private key is given only to the requesting party and the public key is made publicly available (as part of a digital certificate) in a directory that all parties can access. The private key is never shared with anyone or sent across the Internet. You use the private key to decrypt text that has been encrypted with your public key by someone else (who can find out what your public key is from a public directory). Thus, if I send you a message, I can find out your public key (but not your private key) from a central administrator and encrypt a message to you using your public key. When you receive it, you decrypt it with your private key. In addition to encrypting messages (which ensures privacy), you can authenticate yourself to me (so I know that it is really you who sent the message) by using your private key to encrypt a digital certificate. When I receive it, I can use your public key to decrypt it. Here's a table that restates it:
  • To do this Use whose Kind of key
  • Send an encrypted message Use the receiver's Public key
  • Send an encrypted signature Use the sender's Private key
  • Decrypt an encrypted message Use the receiver's Private key
  • Decrypt an encrypted signature (and authenticate the sender) Use the sender's Public key
  • Who Provides the Infrastructure
  • A number of products are offered that enable a company or group of companies to implement a PKI. The acceleration of e-commerce and business-to-business commerce over the Internet has increased the demand for PKI solutions. Related ideas are the virtual private network (VPN) and the IP Security (IPsec) standard. Among PKI leaders are:
  • * RSA, which has developed the main algorithms used by PKI vendors
  • * Verisign, which acts as a certificate authority and sells software that allows a company to create its own certificate authorities
  • * GTE CyberTrust, which provides a PKI implementation methodology and consultation service that it plans to vend to other companies for a fixed price
  • * Xcert, whose Web Sentry product that checks the revocation status of certificates on a server, using the Online Certificate Status Protocol (OCSP)
  • * Netscape, whose Directory Server product is said to support 50 million objects and process 5,000 queries a second; Secure E-Commerce, which allows a company or extranet manager to manage digital certificates; and Meta-Directory, which can connect all corporate directories into a single directory for security management
  • For e-mail, the Pretty Good Privacy (PGP ) product lets you encrypt a message to anyone who has a public key. You encrypt it with their public key and they then decrypt it with their private key. PGP users share a directory of public keys that is called a key ring . (If you are sending a message to someone that doesn't have access to the key ring, you can't send them an encrypted message.) As another option, PGP lets you "sign" your note with a digital signature using your private key. The recipient can then get your public key (if they get access to the key ring) and decrypt your signature to see whether it was really you who sent the message.


  • Python is an interpreted, object-oriented programming language similar to Perl, that has gained popularity because of its clear syntax and readability. Python is said to be relatively easy to learn and portable, meaning its statements can be interpreted in a number of operating systems, including UNIX-based systems, Mac OS, MS-DOS, OS/2, and various versions of Microsoft Windows 98 . Python was created by Guido van Rossum, a former resident of the Netherlands, whose favorite comedy group at the time was Monty Python's Flying Circus. The source code is freely available and open for modification and reuse. Python has a significant number of users.
  • A notable feature of Python is its indenting of source statements to make the code easier to read. Python offers dynamic data type, ready-made class, and interfaces to many system calls and libraries. It can be extended, using the C or C++ language. Python can be used as the script in Microsoft's Active Server Page (ASP) technology. The scoreboard system for the Melbourne (Australia) Cricket Ground is written in Python. Z Object Publishing Environment, a popular Web application server, is also written in the Python language.


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