Understanding the network.pdf - Back to Home

You should also keep in mind that the Layer 3 IP source and destination addresses
have not changed; only the Layer 2 addresses changed as the IP datagram moved
from network to network.
This example walks you through the Layer 2/Layer 3 forwarding process. The same
process is performed by every router that forwards IP datagrams. Also in this
example, you see the use of redundant paths to deliver datagrams. If the link
between Router A and Router D went down, the datagram could have been
delivered over the Router A to Router C, Router C to Router D link.
The Internet Routing Process
Most people's experience with IP routing started with their involvement with the
Internet. The Internet is the common name of the global internetwork of tens of
thousands of distinct inter/intranetworks. The Internet was the driving force behind
the development of various dynamic IP routing protocols. Although the intricacies of
these protocols are covered in Chapter 8, an introduction on the use of dynamic
routing protocols is needed in the context of a chapter on TCP/IP.
When large enterprise and regional networks use centralized routing protocols to
manage their routers, an intradomain routing policy is followed. In instances where
multiple routing protocols are being used to cross-pollinate routing information, an
interdomain routing policy is followed because more than one routing domain is
being used to manage the route discovery across the entire network.
The distinctions between intradomain and interdomain make more sense when you
examine how large and extremely large IP networks are structured. Intradomain
routing is generally used in intranetworks. Most company-wide networks can be
considered intranetworks. An intranetwork can be a single LAN or a collection of
LANs connected by private data links. What defines an intranetwork is its usage.
Intranets have an exclusive set of users, and access is restricted only to that user
community. Most intranets pursue an intradomain routing policy. The term
"intranetwork" is relatively new. The distinction arose as more and more office LANs
and company-wide private data networks began installing access points to the
global Internet.
Intranetworks use Regional Internet Service Providers (RISPs) and/or National
Internet Service Providers (NISPs) to gain access to the Internet. RISPs and NISPs
operate internetworks, which are networks made up of multiple distinct networks
that operate as separate routing domains. These networks connect to a common
internetwork backbone that exists as a routing domain apart from the distinct