25.02.2013 Views

TCP/IP Tutorial and Technical Overview - IBM Redbooks

TCP/IP Tutorial and Technical Overview - IBM Redbooks

TCP/IP Tutorial and Technical Overview - IBM Redbooks

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

► Hosts or networks for which the router has received an ICMP redirect<br />

message.<br />

► A default for all other destinations.<br />

Additional protocols are needed to implement a full-function router. These types<br />

of routers are essential in most networks, because they can exchange<br />

information with other routers in the environment. We review the protocols used<br />

by these routers in Chapter 5, “Routing protocols” on page 171.<br />

There are two types of <strong>IP</strong> routing: direct <strong>and</strong> indirect.<br />

Direct routing<br />

If the destination host is attached to the same physical network as the source<br />

host, <strong>IP</strong> datagrams can be directly exchanged. This is done by encapsulating the<br />

<strong>IP</strong> datagram in the physical network frame. This is called direct delivery <strong>and</strong> is<br />

referred to as direct routing.<br />

Indirect routing<br />

Indirect routing occurs when the destination host is not connected to a network<br />

directly attached to the source host. The only way to reach the destination is<br />

through one or more <strong>IP</strong> gateways. (Note that in <strong>TCP</strong>/<strong>IP</strong> terminology, the terms<br />

gateway <strong>and</strong> router are used interchangeably. This describes a system that<br />

performs the duties of a router.) The address of the first gateway (the first hop) is<br />

called an indirect route in the <strong>IP</strong> routing algorithm. The address of the first<br />

gateway is the only information needed by the source host to send a packet to<br />

the destination host.<br />

In some cases, there may be multiple subnets defined on the same physical<br />

network. If the source <strong>and</strong> destination hosts connect to the same physical<br />

network but are defined in different subnets, indirect routing is used to<br />

communicate between the pair of devices. A router is needed to forward traffic<br />

between subnets.<br />

78 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!