Routing
Data
Data
flow in a network that consists of a single segment is simple. Each
computer that transmits data can broadcast a request over the
network for the MAC address of the destination computer and send the
data to it. However, in networks that have multiple segments, the
data transmission process is more complex. In such environments,
TCP/IP provides for multiple paths between computers and prevents
unnecessary communication from crossing segment boundaries. In an
environment that has connected networks, the source and destination
computers may not be on the same segment. IP determines whether the
destination computer is local or remote in relation to the source
computer. If the destination computer is remote, the data cannot be
sent to it directly. Instead, IP sends it to a router, which then
forwards the packet to its destination. In this section, you will
learn about the role of IP in the routing process and the process by
which data is transmitted across routers.
IP
Routing
Large TCP/IP networks, referred to as internetworks, are broken up
into smaller segments to reduce the amount of communication within
the segment. An
internetwork
is a
network consisting of multiple segments that are connected by
routers. Routers are basically computers with two network adapters
that provide the primary means of joining two or more physically
separated segments. Routers pass IP packets from one network segment
to another. This process of forwarding IP packets is known as
routing. Routers are attached to two or more IP network segments,
enabling packets to be forwarded from one segment to another.
Packet
Delivery
Forwarded IP packets use at least one of two types of delivery,
based on whether the IP packet is forwarded to the final destination
or whether it is forwarded to a router. These two types of delivery
are known as direct and
indirect delivery.
-
Direct delivery occurs when a computer forwards a packet to a
final destination on the same segment. The computer encapsulates
the IP packet in a frame format for the network interface layer
and addresses the packet to the destination’s MAC address.
-
Indirect delivery occurs when a computer forwards a packet to a
router because the final destination is not on the same segment.
The computer encapsulates the IP packet in a frame format for
the network interface layer addressed to the IP router’s MAC
address.
Routing
Table
To
determine where a packet is to be forwarded, routers use routing
tables to send data between network segments. A routing table is
stored in memory and maintains information about other IP networks
and hosts. In addition, a routing table provides information to each
local host about how to communicate with remote networks and hosts.
For each computer on an IP network, you can maintain a routing table
that contains an entry for every other computer or network in
communication with the local computer. However, this is not
practical for large networks and a default router is used to
maintain the routing table. Routing tables can be either static or
dynamic, depending upon the way they are updated. You update a
static routing table manually. Because updating cannot be done
often, the information in the routing table may not be current. On
the other hand, a dynamic routing table is automatically updated as
new information becomes available.
