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networks). Any system with multiple interfaces is called multihomed. A host can also be
multihomed but unless it specifically forwards packets from one interface to another, it is not
called a router. Also, routers need not be special hardware boxes that only move packets around
an internet. Most TCP/IP implementations allow a multihomed host to act as a router also, but
the host needs to be specifically configured for this to happen. In this case we can call the system
either a host (when an application such as FTP or Telnet is being used) or a router (when it's file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/introduc.htm (4 of 20) [12/09/2001 14.46.31] Chapter 1. Introduction forwarding packets from one network to another). We'll use whichever term makes sense given
One of the goals of an internet is to hide all the details of the physical layout of the internet from
the applications. Although this isn't obvious from our two-network internet in Figure 1.3, the
application layers can't care (and don't care) that one host is on an Ethernet, the other on a token
ring, with a router between. There could be 20 routers between, with additional types of physical
interconnections, and the applications would run the same. This hiding of the details is what
makes the concept of an internet so powerful and useful.
Another way to connect networks is with a bridge. These connect networks at the link layer,
while routers connect networks at the network layer. Bridges makes multiple LANs appear to the
upper layers as a single LAN.
TCP/IP internets tend to be built using routers instead of bridges, so we'll focus on routers.
Chapter 12 of [Perlman 1992] compares routers and bridges. 1.3 TCP/IP Layering
There are more protocols in the TCP/IP protocol suite. Figure 1.4 shows some of the additional
protocols that we talk about in this text. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/i...
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