The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

Chapter 11. Virtual LANs: Applications and Concepts

So far, we haven't had much trouble figuring out what a LAN is.^[] We haven't explicitly stated it, but we have a mental image of a LAN as a collection of stations, cables, hubs, and related equipment configured to support communications within a local area. A LAN allows direct Data Link layer communications among the application processes running within those stations attached to the LAN.

^[] We can't imagine how you could get to Chapter 11 without having at least some idea!

A typical medium-to-large organization may have many LANs in place to support large numbers of users and network applications. Each user is typically attached to a single physical LAN; the set of devices connected to a given LAN is defined by the limitations of the equipment employed (for example, the number of ports on a hub) and the cable connections configured by the network administrator (i.e., which devices are connected to which hubs or shared media segments). Once the LAN is installed and configured, its extent is fixed; changing the LAN configuration (i.e., the set of devices and applications that appear on a given LAN) requires a change to those physical connections that created the LAN. The logical connectivity of the LAN is equal to its physical connectivity.

Virtual LAN (VLAN) technology allows you to separate the logical connectivity from the physical connectivity. Users are still connected via physical cables to physical wiring devices, but the ...