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Table Of Contents  The TCP/IP Guide
 9  Networking Fundamentals
      9  Types and Sizes of Networks

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Local Area Networks (LANs), Wireless LANs (WLANs) and Wide Area Networks (WANs) and Variants (CANs, MANs and PANs)
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The Internet, Intranets and Extranets
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Segments, Networks, Subnetworks and Internetworks
(Page 2 of 2)

Segment (Network Segment)

A segment is a small section of a network. In some contexts, a segment is the same as a subnetwork and the terms are used interchangeably. More often, however, the term “segment” implies something smaller than a subnetwork. Networks are often designed so that, for the sake of efficiency, with computers that are related to each other or that are used by the same groups of people put on the same network segment.

This term is notably problematic because it is routinely used in two different ways, especially in discussions related to Ethernet. The earliest forms of Ethernet used coaxial cables, and the coax cable itself was called a “segment”. The segment was shared by all devices connected to it, and became the collision domain for the network (a phrase referring generally to a collection of hardware devices where only one can transmit at a time.)

Each Ethernet physical layer had specific rules about how many devices could be on a segment, how many segments could be connected together, and so on, depending on what sort of network interconnection devices were being used. Devices such as hubs and repeaters were used to extend collision domains by connecting together these segments of cable into wider networks. Over time, the terms “collision domain” and “segment” started to be used interchangeably. Thus today a “segment” can refer either to a specific piece of cable, or to a collection of cables connected electrically that represent a single collision domain.

Note: As if that potential ambiguity in the use of the word “segment” isn’t bad enough, it also has another, totally unrelated meaning: it is the name of the messages sent in the Transmission Control Protocol!


Internetwork (or Internet)

Most often, this refers to a larger networking structure that is formed by connecting together smaller ones. Again, the term can have either a generic or a specific meaning, depending on context. In some technologies, an internetwork is just a very large network that has networks as components. In others, a network is differentiated from an internetwork based on how the devices are connected together.

An important example of the latter definition is TCP/IP, where a network usually refers to a collection of machines that are linked at layer two of the OSI Reference Model, using technologies like Ethernet or Token Ring and interconnection devices such as hubs and switches. An internetwork is formed when these networks are linked together at layer three, using routers that pass Internet Protocol datagrams between networks. Naturally, this is highly simplified, but in studying TCP/IP you should keep this in mind when you encounter the terms “network” and “internetwork”.

Note: The shorter form of the word internetwork (“internet”) is often avoided by people who wish to avoid confusion with the proper noun form (“The Internet”). The latter of course refers only to the well-known global internetwork of computers and all the services it provides. I personally try to use the word “internetwork” most of the time in this Guide instead of “internet”, for this very reason.


Key Concept: Several terms are often used to describe the relative sizes of networks and parts of networks. The most basic term is network itself, which can refer to most anything, but often means a set of devices connected using an OSI layer two technology. A subnetwork is a part of a network (or internetwork), as is a segment, though the latter often has a more specific meaning in certain technologies. An internetwork refers either generically to a very large network, or specifically to a set of layer-two networks connected using routers at layer three.



Previous Topic/Section
Local Area Networks (LANs), Wireless LANs (WLANs) and Wide Area Networks (WANs) and Variants (CANs, MANs and PANs)
Previous Page
Pages in Current Topic/Section
1
2
Next Page
The Internet, Intranets and Extranets
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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