Page 143 - DCAP207_NETWORKS_DCAP406_COMPUTER_NETWORKS

Computer Networks/Networks

Notes closer to its final destination to which the datagram should be sent. To determine the next hop, the
routers maintain a set of information to enable mapping between different networks IDs and the
other routers to which it is connected. This information is contained in a data structure known as
routing table. The entries in the routing table facilitate details of subnetwork or host. Thus, when
a router receives a datagram, it examines the destination IP address of datagram against the
routing entries in its table to determine where to send the datagram, and then sends it on its next
hop. The next hop is a technique to enable router to take fast decision what to do with datagrams
due to the fewer the entries in this table. The classless inter domain routing (CIDR) aggregates
routes into supernets to reduce router table size. In brief the next hop routing is the technique to
simply reduce the contents of a routing table, which holds information that leads to the next hop
instead of holding information about the complete route.
Network specific routing: The network specific routing is also a technique to reduce the routing
table and simplify the searching process. As the name implies, this technique allows one entry
only to define the address of the network itself to which many hosts are connected. Thus, the
network specific routing does not involve an entry for each and every host connected to the
same physical network and treat all hosts connected to same network as one single entity. For
example, if there are 500 hosts attached to the same network, only one entry exists in the routing
table instead of 500 entries.
Host specific routing: It is considered inverse of network-specific routing in which each destination
host addresses are given in the routing table. Thus it is not as fast as next hop and network
specific routing, however network administrator has greater control over routing.
Default routing: It is another technique to simplify routing in which a host is connected to two
routers in a network. One router is used to route the packets to the host connected to another
network and for the rest of the Internet another router is used.

Routing are grouped into two classes. They are non-adaptive and adaptive algorithms.
Non-adaptive algorithms or static routing are independent of the volume of the current traffic
and topology. They decide the route to which a datagram is to sent off-line. The route is computed
in advance and downloaded to the routers when the network is booted. Thus, routing information
is manually specified. It provides fixed route information to each router. If there is no change in
route, it is made manually. This procedure is also called static routing.
Adaptive algorithms or dynamic routing are capable of changing their routing decisions to
reflect changes in the topology and the traffic. Routers automatically update routing information
when changes are made to the network configuration. It is convenient, as it does not involve
human intervention in case of changes to the network configuration. Its disadvantage, however,
is that the overhead required to send configuration change information can be a heavy burden.
They are also known as dynamic routing. To update information in the routing table, it uses one
of the dynamic routing protocol such as OSPF or BGP or etc.

9.2.1 Routing Table

Each router on the network maintains a routing table in memory that may be simple or complex.
In the simplest form the table is consisted pairs of IP addresses. When the originating station
concludes that the intended destination is, itself, directly reachable, the frame is sent directly to
the destination IP address in the frame. However, it may not be necessary to send it to a router
if the sender finds that it is on the same subnetwork as the destination. When the masked
destination address is compared against the available entries in the table and it is found that the
routing table does not possess any matching lookup value. In such a situation, a special address
appears in the routing table called the Default Gateway Address. The routing decisions are
based on the following points:

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