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Quick Reference Guide to the Ethernet System

1.9 Extending Ethernets with Hubs

Ethernet was designed to be easily expandable to meet the networking needs of a given site. To help extend Ethernet systems, networking vendors sell devices that provide multiple Ethernet ports. These devices are known as hubs since they provide the central portion, or hub, of a media system.

There are two major kinds of hub: repeater hubs and switching hubs. As we've seen, each port of a repeater hub links individual Ethernet media segments together to create a larger network that operates as a single Ethernet LAN. The total set of segments and repeaters in the Ethernet LAN must meet the round trip timing specifications. The second kind of hub provides packet switching, typically based on bridging ports as described in Chapter 15.

The important thing to know at this point is that each port of a packet switching hub provides a connection to an Ethernet media system that operates as a separate Ethernet LAN. Unlike a repeater hub whose individual ports combine segments together to create a single large LAN, a switching hub makes it possible to divide a set of Ethernet media systems into multiple LANs that are linked together by way of the packet switching electronics in the hub. The round trip timing rules for each LAN stop at the switching hub port. This allows you to link a large number of individual Ethernet LANs together.

A given Ethernet LAN can consist of merely a single cable segment linking some number of computers, or it may consist of a repeater hub linking several such media segments together. Whole Ethernet LANs can themselves be linked together to form extended network systems using packet switching hubs. While an individual Ethernet LAN may typically support anywhere from a few up to several dozen computers, the total system of Ethernet LANs linked with packet switches at a given site may support many hundreds or thousands of machines.

Quick Reference Guide to the Ethernet System - 04 SEP 95
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