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Glossary: CCNA & General Networking Terms 


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The unit of data sent across a network. "Packet" a generic term used to describe unit of data at all levels of the protocol stack, but it is most correctly used to describe application data units. See also: datagram, frame. [Source: RFC1392]
Packet InterNet Groper (PING)
A program used to test reachability of destinations by sending them an ICMP echo request and waiting for a reply. The term is used as a verb: "Ping host X to see if it is up!" See also: Internet Control Message Protocol. [Source: RFC1208]
Packet Switch Node (PSN)
A dedicated computer whose purpose is to accept, route and forward packets in a packet switched network. See also: packet switching, router. [Source: NNSC]
packet switching
A communications paradigm in which packets (messages) are individually routed between hosts, with no previously established communication path. See also: circuit switching, connection- oriented, connectionless. [Source: RFC1392]
Packet Assembler Disassembler; the hardware or software interface between a user's terminal and a packet-switching network. A PAD assembles the user's input characters into packets for network transmission, and disassembles packets of output characters into their component characters for output on the terminal. The PAD facility may run on a host computer or on a dedicated processor (such as the JNT-PAD).
Particle Physics Network Co-ordinating Group (PPNCG)
The Particle Physics Network Co-ordinating Group (PPNCG) is responsible for the networking interests of the UK particle physics community. Formally it is a sub-committee of the Particle Physics Committee (PPC) of the Particle Physics and Astronomy Research Council (PPARC). PPNCG is also a Affiliated Group of the JANET National User Group, through which the views of the users on JANET and UKERNA services are presented to the JISC Committee on Networking. See also: JANET National User Group.
Protocol Control Information. The protocol information added by an OSI entity to the service data unit passed down from the layer above, all together forming a Protocol Data Unit (PDU). [Source: RFC1208]
Public Domain
See: Protocol Data Unit
See: Privacy Enhanced Mail
See: Pretty Good Privacy
PHP: Hypertext Processor. An HTML-embedded scripting language used to create dynamic web pages by running scripts on the web server and embedding the results in web pages. Available from
Physical Layer
The OSI layer that provides the means to activate and use physical connections for bit transmission. In plain terms, the Physical Layer provides the procedures for transferring a single bit across a Physical Media. [Source: RFC1208]
Physical Media
Any means in the physical world for transferring signals between OSI systems. Considered to be outside the OSI Model, and therefore sometimes referred to as "Layer 0." The physical connector to the media can be considered as defining the bottom interface of the Physical Layer, i.e., the bottom of the OSI Reference Model. [Source: RFC1208]
See: Packet INternet Groper
Pink Book
the document describing the implementation of X25 protocol level 3 over a Connection Orientated Network Service based on lower layers of Ethernet protocol. This enables interconnectivity between e.g. Ethernet based PCs and X25 based hosts, and supports Coloured Book protocols for file transfer and terminal access, e.g. with the Rainbow package. Since it is based on ISO standards, Pink Book is not strictly a "Coloured Book" in the JANET usage.
Point Of Presence (POP)
A site where there exists a collection of telecommunications equipment, usually digital leased lines and multi-protocol routers. [Source: RFC1392]
Point-to-Point Protocol (PPP)
The Point-to-Point Protocol, defined in RFC 1661, provides a method for transmitting packets over serial point-to-point links. There are many other RFCs which define extensions to the basic protocol. See also: Serial Line IP. [Source: FYI4]
Connecting to another system to check for things like mail or news.
See: Post Office Protocol and Point Of Presence
A port is a transport layer demultiplexing value. Each application has a unique port number associated with it. See also: Transmission Control Protocol, User Datagram Protocol. [Source: RFC1392]
A web site integrating many facilities within one user interface. Only of value if the infrastructure for personal information databases, authentication and authorisation methods, group communication, business processes and timely content have already been engineered for harmonious web access. Driver software to address a multiplicity of browser devices, from PCs and personal digital assistants to mobile phones and interactive TV, should make content available in many environments.
Promoting Conference for OSI. The OSI "800-pound gorilla" in Japan. Consists of executives from the six major Japanese computer manufacturers and Nippon Telephone and Telegraph. They set policies and commit resources to promote OSI. [Source: RFC1208]
Post Office Protocol (POP)
A protocol designed to allow single user hosts to read mail from a server. Version 3, the most recent and most widely used, is defined in RFC 1725. See also: Electronic Mail. [Source: RFC1983]
Postal Telegraph and Telephone (PTT)
Outside the USA, PTT refers to a telephone service provider, which is usually a monopoly, in a particular country. [Source: RFC1392]
The person responsible for taking care of electronic mail problems, answering queries about users, and other related work at a site. See also: Electronic Mail. [Source: ZEN]
See: Particle Physics Network Co-ordinating Group
See: Point-to-Point Protocol
Presentation Address
See OSI Presentation Address.
Presentation Layer
The OSI layer that determines how Application information is represented (i.e., encoded) while in transit between two end systems. [Source: RFC1208]
Pretty Good Privacy (PGP)
A program, developed by Phil Zimmerman, which cryptographically protects files and electronic mail from being read by others. It may also be used to digitally sign a document or message, thus authenticating the creator. See also: encryption, Data Encryption Standard, RSA. [Source: RFC1983]
Privacy Enhanced Mail (PEM)
Internet email which provides confidentiality, authentication and message integrity using various encryption methods. See also: Electronic Mail, encryption. [Source: RFC1392]
Private Management Domain. An X.400 Message Handling System private organization mail system. Example:- NASAmail. See ADMD. [Source: RFC1208]
A distributed filesystem which provides the user with the ability to create multiple views of a single collection of files distributed across the Internet. Prospero provides a file naming system, and file access is provided by existing access methods (e.g., anonymous FTP and NFS). The Prospero protocol is also used for communication between clients and servers in the archie system. See also: anonymous FTP, archie, archive site, Gopher, Network File System, Wide Area Information Servers. [Source: RFC1392]
A formal description of message formats and the rules two computers must follow to exchange those messages. Protocols can describe low-level details of machine-to-machine interfaces (e.g., the order in which bits and bytes are sent across a wire) or high-level exchanges between allocation programs (e.g., the way in which two programs transfer a file across the Internet). [Source: MALAMUD]
protocol converter
A device/program which translates between different protocols which serve similar functions (e.g., TCP and TP4). [Source: RFC1392]
Protocol Data Unit (PDU)
"PDU" is internationalstandardscomitteespeak for packet. See also: packet. [Source: RFC1392]
protocol stack
A layered set of protocols which work together to provide a set of network functions. See also: layer, protocol. [Source: RFC1392]
The mechanism whereby one system "fronts for" another system in responding to protocol requests. Proxy systems are used in network management to avoid having to implement full protocol stacks in simple devices, such as modems. [Source: RFC1208]
proxy ARP
The technique in which one machine, usually a router, answers ARP requests intended for another machine. By "faking" its identity, the router accepts responsibility for routing packets to the "real" destination. Proxy ARP allows a site to use a single IP address with two physical networks. Subnetting would normally be a better solution. See also: Address Resolution Protocol [Source: RFC1208]
See: Packet Switch Node.
the Packet SwitchStream of British Telecom. PSS includes a UK packet-switching network service as well as a set of PADs in various cities. Now part of Global Network Services.
See: Postal, Telegraph and Telephone
Public Telecommunications Operator. See also: Postal, Telegraph and Telephone.
Name record for a host of given address, for reverse lookup, in Domain Name Service.


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