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NAP - Network Access Point:

  • See Network Access Point.

NAS - Network Attached Storage:

  • See Network Attached Storage.

NAT - Network Address Translation:

  • ee Network Address Translation.

Nation Security Agency - NSA:

  • The National Security Agency (NSA) is the official U.S. cryptologic (the science of cryptographic design and decryption) organization. Under a directive (revised version of the National Security Council Intelligence Directive (NSCID) No. 9) from President Truman and the National Security Council, the NSA was formed in November of 1952. The organization exists to protect national communications systems integrity, and to seek information about foreign adversaries' secret communications. The NSA works in close conjunction with the Central Security Service (CSS), which was established by Presidential Directive in 1972 to promote full partnership between the NSA and the cryptologic elements of the Armed Forces. The director of NSA/CSS, in accordance with a Department of Defense (DoD) directive, must be a high-ranking (at least 3 stars) commissioned officer of the military services.
  • Although the organization's number of employees (as well as its budget) falls into the category of "classified information," the NSA lists among its workforce: analysts, engineers, physicists, linguists, computer scientists, researchers, customer relations specialists, security officers, data flow experts, managers, administrative and clerical assistants; it also claims to be the largest employer of mathematicians in the U.S., and possibly worldwide. NSA/CSS mathematicians perform the Agency's two critical functions: they design cryptographic systems to protect U.S. communications and search for weaknesses in the counterpart systems of U.S. adversaries.
  • NSA denies rumours in circulation claiming that it has an unlimited "black budget" - undisclosed even to other government agencies. Nevertheless, the Agency admits that, if it were judged as a corporation, it would rank in the top ten percent of Fortune 500 companies.

NDS - Network Access Point:

  • See Network Access Point.

NetBEUI - NetBIOS Extended User Interface:

  • NetBIOS Extended User Interface (NetBEUI) is a new, extended version of NetBIOS, the program that lets computers communicate within a local area network. NetBEUI (pronounced net-BOO-ee ) formalizes the frame format (or arrangement of information in a data transmission) that was not specified as part of NetBIOS. NetBEUI was developed by IBM for its LAN Manager product and has been adopted by Microsoft for its Windows NT, LAN Manager, and Windows for Workgroups products. Hewlett-Packard and DEC use it in comparable products.
  • NetBEUI is the best performance choice for communication within a single LAN. Because, like NetBIOS, it does not support the routing of messages to other networks, its interface must be adapted to other protocols such as Internetwork Packet Exchange or TCP/IP . A recommended method is to install both NetBEUI and TCP/IP in each computer and set the server up to use NetBEUI for communication within the LAN and TCP/IP for communication beyond the LAN

NetBIOS - Network Basic Input/Output System:

  • Network Basic Input/Output System (NetBIOS) is a program that allows applications on different computers to communicate within a local area network (LAN ). It was created by IBM for its early PC Network, was adopted by Microsoft, and has since become a de facto industry standard. NetBIOS is used in Ethernet, token ring, and Windows NT networks. It does not in itself support a routing mechanism so applications communicating on a wide area network (WAN) must use another "transport mechanism" (such as Transmission Control Protocol) rather than or in addition to NetBIOS.
  • NetBIOS frees the application from having to understand the details of the network, including error recovery (in session mode). A NetBIOS request is provided in the form of a Network Control Block (NCB) which, among other things, specifies a message location and the name of a destination.
  • NetBIOS provides the session and transport services described in the Open Systems Interconnection (OSI ) model. However, it does not provide a standard frame or data format for transmission. A standard frame format is provided in the NetBIOS Extended User Interface (NetBEUI).
  • NetBIOS provides two communication modes: session or datagram. Session mode lets two computers establish a connection for a "conversation," allows larger messages to be handled, and provides error detection and recovery. Datagram mode is "connectionless" (each message is sent independently), messages must be smaller, and the application is responsible for error detection and recovery. Datagram mode also supports the broadcast of a message to every computer on the LAN.

Network Access Point - NAP:

  • In the United States, a network access point (NAP) is one of several major Internet interconnection points that serve to tie all the Internet access providers together so that, for example, an AT&T user in Portland, Oregon can reach the Web site of a Bell South customer in Miami, Florida. Originally, four NAPs - in New York, Washington, D.C., Chicago, and San Francisco - were created and supported by the National Science Foundation as part of the transition from the original U.S. government-financed Internet to a commercially operated Internet. Since that time, several new NAPs have arrived, including WorldCom's "MAE West" site in San Jose, California and ICS Network Systems' "Big East."
  • The NAPs provide major switching facilities that serve the public in general. Using companies apply to use the NAP facilities and make their own intercompany peering arrangements. Much Internet traffic is handled without involving NAPs, using peering arrangements and interconnections within geographic regions. The vBNS network, a separate network supported by the National Science Foundation for research purposes, also makes use of the NAPs.

Network Attached Storage - NAS:

  • Network-attached storage (NAS) is hard disk storage that is set up with its own network address rather than being attached to the department computer that is serving applications to a network's workstation users. By removing storage access and its management from the department server, both application programming and files can be served faster because they are not competing for the same processor resources. The network-attached storage device is attached to a local area network (typically, an Ethernet network) and assigned an IP address. File requests are mapped by the main server to the NAS file server.
  • Network-attached storage consists of hard disk storage, including multi-disk RAID systems, and software for configuring and mapping file locations to the network-attached device. Network-attached storage can be a step toward and included as part of a more sophisticated storage system known as a storage area network (SAN).
  • NAS software can usually handle a number of network protocols, including Microsoft's Internetwork Packet Exchange and NetBEUI, Novell's Netware Internetwork Packet Exchange, and Sun Microsystems' Network File System. Configuration, including the setting of user access priorities, is usually possible using a Web browser.

Network Address Translation - NAT:

  • Network Address Translation (NAT) is the translation of an Internet Protocol address (IP address) used within one network to a different IP address known within another network. One network is designated the inside network and the other is the outside . Typically, a company maps its local inside network addresses to one or more global outside IP addresses and unmaps the global IP addresses on incoming packets back into local IP addresses. This helps ensure security since each outgoing or incoming request must go through a translation process that also offers the opportunity to qualify or authenticate the request or match it to a previous request. NAT also conserves on the number of global IP addresses that a company needs and it lets the company use a single IP address in its communication with the world.
  • NAT is included as part of a router and is often part of a corporate firewall . Network administrators create a NAT table that does the global-to-local and local-to-global IP address mapping. NAT can also be used in conjunction with policy routing. NAT can be statically defined or it can be set up to dynamically translate from and to a pool of IP addresses. Cisco's version of NAT lets an administrator create tables that map:
  • * A local IP address to one global IP address statically
  • * A local IP address to any of a rotating pool of global IP addresses that a company may have
  • * A local IP address plus a particular TCP port to a global IP address or one in a pool of them
  • * A global IP address to any of a pool of local IP addresses on a round-robin basis
  • NAT is described in general terms in RFC 1631. which discusses NAT's relationship to Classless Interdomain Routing (CIDR ) as a way to reduce the IP address depletion problem. NAT reduces the need for a large amount of publicly known IP addresses by creating a separation between publicly known and privately known IP addresses. CIDR aggregates publicly known IP addresses into blocks so that fewer IP addresses are wasted. In the end, both extend the use of IPv4 IP addresses for a few more years before IPv6 is generally supported.

Network Information System - NIS:

  • Network Information System (NIS) is a network naming and administration system for smaller networks that was developed by Sun Microsystems. NIS+ is a later version that provides additional security and other facilities. Using NIS, each host client or server computer in the system has knowledge about the entire system. A user at any host can get access to files or applications on any host in the network with a single user identification and password. NIS is similar to the Internet's domain name system (DNS) but somewhat simpler and designed for a smaller network. It's intended for use on local area networks.
  • NIS uses the client/server model and the Remote Procedure Call (RPC ) interface for communication between hosts. NIS consists of a server, a library of client programs, and some administrative tools. NIS is often used with the Network File System (NFS). NIS is a UNIX-based program.
  • Although Sun and others offer proprietary versions, most NIS code has been released into the public domain and there are freeware versions available. NIS was originally called Yellow Pages but because someone already had a trademark by that name, it was changed to Network Information System. It is still sometimes referred to by the initials: "YP". Sun offers NIS+ together with its NFS product as a solution for Windows PC networks as well as for its own workstation networks.

Network Layer:

  • In the Open Systems Interconnection (OSI ) communications model, the Network layer knows the address of the neighboring nodes in the network, packages output with the correct network address information, selects routes and Quality of Service, and recognizes and forwards to the Transport layer incoming messages for local host domains. Among existing protocol that generally map to the OSI network layer are the Internet Protocol (IP) part of TCP/IP and NetWare IPX/SPX. Both IP Version 4 and IP Version 6 (IPv6) map to the OSI network layer.

Network News Transfer Protocol - NNTP:

  • Network News Transfer Protocol (NNTP) is the predominant protocol used by computer clients and servers for managing the notes posted on Usenet newsgroups. NNTP replaced the original Usenet protocol, UNIX-to-UNIX Copy Protocol (UUCP ) some time ago. NNTP servers manage the global network of collected Usenet newsgroups and include the server at your Internet access provider. An NNTP client is included as part of a Netscape, Internet Explorer, Opera, or other Web browser or you may use a separate client program called a newsreader.

Network Time Protocol - NTP:

  • Network Time Protocol (NTP) is a protocol that is used to synchronize computer clock times in a network of computers. Developed by David Mills at the University of Delaware, NTP is now an Internet standard. In common with similar protocols, NTP uses Coordinated Universal Time (UTC) to synchronize computer clock times to a millisecond, and sometimes to a fraction of a millisecond.
  • Accurate time across a network is important for many reasons; even small fractions of a second can cause problems. For example, distributed procedures depend on coordinated times to ensure that proper sequences are followed. Security mechanisms depend on coordinated times across the network. File system updates carried out by a number of computers also depend on synchronized clock times. Air traffic control systems provide a graphic illustration of the need for coordinated times, since flight paths require very precise timing (imagine the situation if air traffic controller computer clock times were not synchronized).
  • UTC time is obtained using several different methods, including radio and satellite systems. Specialized receivers are available for high-level services such as the Global Positioning System (GPS ) and the governments of some nations. However, it is not practical or cost-effective to equip every computer with one of these receivers. Instead, computers designated as primary time servers are outfitted with the receivers and they use protocols such as NTP to synchronize the clock times of networked computers. Degrees of separation from the UTC source are defined as strata. A radio clock (which receives true time from a dedicated transmitter or satellite navigation system) is stratum-0; a computer that is directly linked to the radio clock is stratum-1; a computer that receives its time from a stratum-1 computer is stratum-2, and so on.
  • The term NTP applies to both the protocol and the client/server programs that run on computers. The programs are compiled by the user as an NTP client, NTP server, or both. In basic terms, the NTP client initiates a time request exchange with the time server. As a result of this exchange, the client is able to calculate the link delay, its local offset, and adjust its local clock to match the clock at the server's computer. As a rule, six exchanges over a period of about five to 10 minutes are required to initially set the clock. Once synchronized, the client updates the clock about once every 10 minutes, usually requiring only a single message exchange. redundant servers and varied network paths are used to ensure reliability and accuracy. In addition to client/server synchronization, NTP also supports broadcast synchronization of peer computer clocks. NTP is designed to be highly fault-tolerant and scalable.

NFS - Network File System:

NIS - Network Information System:

  • See Network Information System.

NNTP - Network News Transfer Protocol:

  • See Network News Transfer Protocol.

NSA - National Security Agency:

  • See National Security Agency.

nslookup:

  • nslookup is the name of a program that lets an Internet server administrator or user enter a host name (for example, "whatis.com") and find out the corresponding IP address. It will also do reverse name lookup and find the host name for an IP address you specify.
  • For example, if you entered "whatis.com", you would receive as a response our IP address, which happens to be :
  • 216.34.126.200
  • Or if you entered "216.34.126.200", it would return "whatis.com"
  • .
  • nslookup sends a domain name query packet to a designated (or defaulted) domain name system (DNS ) server. Depending on the system you are using, the default may be the local DNS name server at your service provider, some intermediate name server, or the root server system for the entire domain name system hierarchy.
  • Using the Linux and possibly other versions of nslookup , you can locate other information associated with the host name or IP address, such as associated mail services. nslookup is often included with a UNIX -based operating system. Free versions exist that can be added to Windows operating systems. A more limited alternative to nslookup for looking up an IP address is the ping command.

Novell Directory Services - NDS:

  • Novell Directory Services (NDS) is a popular software product for managing access to computer resources and keeping track of the users of a network, such as a company's intranet, from a single point of administration. Using NDS, a network administrator can set up and control a database of users and manage them using a directory with an easy-to-use graphical user interface (GUI ). Users of computers at remote locations can be added, updated, and managed centrally. Applications can be distributed electronically and maintained centrally.
  • NDS can be installed to run under Windows NT, Sun Microsystem's Solaris, and IBM's OS/390 as well as under Novell's own NetWare so that it can be used to control a multi-platform network. NDS is generally considered an industry benchmark against which other products, such as Microsoft's Active Directory, must compete.
  • Lucent Technologies plans to integrate NDS into its own QIP product, which automatically maintains directory information about network Internet Protocol (IP) domain name and IP address and Dynamic Host Configuration Protocol (DHCP ) information. Administrators will be able to set up primary and secondary directories and automate immediate switchover to the backup directory in the event of a server failure.

NTFS - NT File System:

  • NT File System (NTFS) and sometimes New Technology File System) is the file system that the Windows NT operating system uses for storing and retrieving files on a hard disk. NTFS is the Windows NT equivalent of the Windows 95 file allocation table (FAT) and the OS/2 High Performance File System (HPFS). However, NTFS offers a number of improvements over FAT and HPFS in terms of performance, extendibility, and security.
  • <a> <img></a> Notable features of NTFS include:
  • * Use of a B-tree directory scheme to keep track of file clusters
  • * Information about a file's clusters and other data is stored with each cluster, not just a governing table (as FAT is)
  • * Support for very large files (up to 2 to the 64th power or approximately 16 billion bytes in size)
  • * An access control list (ACL) that lets a server administrator control who can access specific files
  • * Integrated file compression
  • * Support for names based on Unicode
  • * Support for long file names as well as "8 by 3" names
  • * Data security on both removable and fixed disks
  • How NTFS Works
  • When a hard disk is formatted (initialized), it is divided into partitions or major divisions of the total physical hard disk space. Within each partition, the operating system keeps track of all the files that are stored by that operating system. Each file is actually stored on the hard disk in one or more clusters or disk spaces of a predefined uniform size. Using NTFS, the sizes of clusters range from 512 bytes to 64 kilobytes. Windows NT provides a recommended default cluster size for any given drive size. For example, for a 4 GB (gigabyte ) drive, the default cluster size is 4 KB (kilobytes). Note that clusters are indivisible. Even the smallest file takes up one cluster and a 4.1 KB file takes up two clusters (or 8 KB) on a 4 KB cluster system.
  • The selection of the cluster size is a trade-off between efficient use of disk space and the number of disk accesses required to access a file. In general, using NTFS, the larger the hard disk the larger the default cluster size, since it's assumed that a system user will prefer to increase performance (fewer disk accesses) at the expense of some amount of space inefficiency.
  • When a file is created using NTFS, a record about the file is created in a special file, the Master File Table (MFT). The record is used to locate a file's possibly scattered clusters. NTFS tries to find contiguous storage space that will hold the entire file (all of its clusters). Each file contains, along with its data content, a description of its attributes (its metadata).

NTP - Network Time Protocol:

  • See Network Time Protocol.

Nyquist Theorem:

  • The Nyquist Theorem, also known as the sampling theorem, is a principle that engineers follow in the digitization of analog signals. For analog-to-digital conversion (ADC) to result in a faithful reproduction of the signal, slices, called samples, of the analog waveform must be taken frequently. The number of samples per second is called the sampling rate or sampling frequency.
  • Any analog signal consists of components at various frequencies. The simplest case is the sine wave , in which all the signal energy is concentrated at one frequency. In practice, analog signals usually have complex waveforms, with components at many frequencies. The highest frequency component in an analog signal determines the bandwidth of that signal. The higher the frequency, the greater the bandwidth, if all other factors are held constant.
  • Suppose the highest frequency component, in hertz, for a given analog signal is fmax. According to the Nyquist Theorem, the sampling rate must be at least 2fmax , or twice the highest analog frequency component. The sampling in an analog-to-digital converter is actuated by a pulse generator (clock). If the sampling rate is less than 2fmax, some of the highest frequency components in the analog input signal will not be correctly represented in the digitized output. When such a digital signal is converted back to analog form by a digital-to-analog converter, false frequency components appear that were not in the original analog signal. This undesirable condition is a form of distortion called aliasing.





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