Figure 1. Seven-layer ISO protocol model.
Malicious software such as Trojan horses and worms can attack LANs. In fact, the physical distribution of any network increases the difficulty of protection. Malicious software is discussed in Essay 4. LANs connect computers, terminals, workstations, and other data terminal equipment (DTE). In this essay we will use “DTE” to refer to whatever is connected to the LAN when it is not important what function it serves. The distinction between a personal computer and a workstation is not important for the purposes of this essay. Let’s start with a functional definition. A LAN is a private communications facility, usually owned by the organization that uses it. The cost of
using the LAN is fixed, independent of level of usage. LANs provide an opportunity for the owning organization to customize its communications capabilities in many ways, such as carrying audio, video, and data traffic; providing multiple simultaneous connections; and providing security services. A LAN generally serves a limited geographic area, such as a single building or a campus, providing a high communications rate or bandwidth and common access to a wide range of devices and services. In general, LANs may be partitioned or