An Introduction to Internetworking
Internetworking is the functional interconnection of two or more networks; the resources of each
individual network become available to the users and machines connected to the other networks.
Internetworking requires a combination of technologies, addressing, and communications protocols. All
these must be understood and adhered to universally throughout the internetwork. Many different devices
can be used to build internetworks, including switches, bridges, and routers. Although the boundaries
between these devices had historically been very distinct, technological advances have blurred these
distinctions. Routers offered the unique capability to discover paths (or routes) through large and
complex internetworks. More importantly, routers could compare different routes through a network to
find the most efficient one between any given points in the network. Routing is still critical to
internetworking. Routing is no longer a function of just standalone routers, however. Routing can be
performed by computers attached to local area networks (LANs) or even by LAN switches!
This chapter introduces the concept of internetworking, examines the role of the router in an
internetwork, and defines some of the more salient terms and concepts that are reinforced throughout this
book. Given that internetworking is best understood through the use of a layered model, this chapter
begins with an overview of the most common of such models: the Open Systems Interconnect (OSI)
reference model. This forms the context for an examination of the mechanics of passing data between
internetworked computers, as well as between networks, using the Internet Protocol (IP).
The OSI Reference Model
The International Organization for Standardization (ISO) developed the OSI reference model to facilitate
the open interconnection of computer systems. An open interconnection is one that can be supported in a
multivendor environment. The reference model identifies and stratifies into logically ordered layers all
the functions required to establish, use, define, and dismantle a communications session between two
computers without regard for those computers' manufacturer or architecture.
Implicit in this definition of the OSI reference model is the assumption that an unknown quantity of
distance and networking gear separate the two communicating devices. Consequently, the model defines
mechanisms for passing data between two machines that share the same LAN or WAN. More
importantly, the model identifies functions that allow two machines that are halfway around the world
from each other with no direct network connections to pass data between themselves.
Note The Dawn of Openness
Today, the OSI reference model is sometimes regarded as logical but trite and not particularly useful.
When it was developed almost 20 years ago, however, it was viewed as radical if not outright
revolutionary. At that time, computer manufacturers locked customers into proprietary, single- vendor
architectures. The price of the convenience of such one-stop shopping was a very long-term commitment
to a single supplier. Frequently, this resulted in inflated prices, forced upgrades, and other unpleasantries
that consumers had little choice but to endure.
The notion of functional modularity, or layering, seemed antithetical to the conventional wisdom of that
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