INTERNETWORK.ING "

INTERNETWORKING "

In most cases, a LAN or WAN is not an isolated entity. An organization may have
more than one type of LAN at a given site to satisfy a spectrum of needs. An organization
may have multiple LANs of the same type at a given site to accommodate
performance or security requirements. And an organization may have LANs at various
sites and need them to be interconnected via WANs for central control of distributed
information exchange.
 lists some commonly used terms relating to the interconnection of networks,
or internetworking. An interconnected set of networks, from a user's point of
view, may appear simply as a larger network. However, if each of the constituent networks
retains its identity, and special mechanisms are needed for communicating
across multiple networks, then the entire configuration is often referred to as an
internet. The most important example of an internet is referred to simply as the Internet.
As the Internet has evolved from its modest beginnings as a research-oriented

packet-switching network, it has served as the basis for the development of internetworking
technology and as the model for private internets within organizations. These
latter are also referred to as intranets.
Each constituent subnetwork in an internet supports communication
among the devices attached to that subnetwork; these devices are referred to as
end systems (ESs). In addition, subnetworks are connected by devices referred to
in the ISO documents as intermediate systems (ISs). ISs provide a communications
path and perform the necessary relaying and routing functions so that
data can be exchanged between devices attached to different subnetworks in
the internet.
Two types of ISs of particular interest are bridges and routers. The differences
between them have to do with the types of protocols used for the internetworking
logic. In essence, a bridge operates at layer 2 of the OSI seven-layer architecture
and acts as a relay of frames between like networks. A router operates at layer 3
of the OSI architecture and routes packets between potentially different
networks. Both the bridge and the router assume that the same upper-layer protocols
are in use.
The roles and functions of routers were introduced in the context of IP earlier
in this chapter. However, because of the importance of routers in the overall networking
scheme, it is worth providing additional comment in this section.