Page 144 - DCAP207_NETWORKS_DCAP406_COMPUTER_NETWORKS

Unit 9: Network Layer

1. The destination IP address and router IP are masked to determine if the incoming packet Notes
is to be forwarded to another network or not. If the results are the same, it indicates that a
packet is for the same subnet as the destination. The frame is then forwarded directly to
the data link address of the destination.
2. When the result is not same, it indicates that the destination is not on the same subnet. The
routing table is checked to find out if the exact, complete, 32-bit destination address is
specified which is referred to as a host specific routing. If host specific route is specified,
the frame is transmitted to the IP destination indicated in the table that implies that this
destination is the next router in line on the way to the destination.

3. When host specific route is not found in the routing table then the masked address is used
to lookup key in the routing table to examine whether the network/subnetwork is specified
in the table. If it is specified, the frame is sent to the IP address specified in the table that
implies that this is the IP address of the next router in line.
4. When both the conditions 2 and 3 given above fail, the frame is forwarded to the address
specified as the target for the Default Gateway.
5. In a situation when no default gateway is specified, it is assumed that all unspecified
destinations are directly reachable. The physical address of the destination IP station is
resolved and the frame is forwarded directly to the destination. This is sometimes called
as activating Proxy ARP.
It is evident from the above that routing table requires at least four entries like mask, destination
address, next hop address and interface.

Self Assessment

State whether the following statements are true or false:
1. The routing algorithm enables the network layer to decide to which output line an incoming
packet should be forwarded.

2. There are three different types of communication links.
3. Connection-oriented services are useful when the user has a constant stream of data to
transmit.

4. A connection-oriented service is equivalent to the postal system.
5. In connection-oriented service, the packets are routed along the same path, known as a
virtual circuit.

9.3 Routing Protocols

Routers are used to connect different networks, determine which path it should take and forward
IP traffic. These informations are obtained at routers by performing per-packet processing in
which IP header of the packet is checked to make routing decisions based on destination IP
address and the current state of the network connectivity. The network connectivity and routing
information is constantly maintained by the router to accurately forward the packets. The packet
is eventually passed though many routers before reaching to the destination. The routing table
at each router in the way of a packet reaching to its indented destination specifies the optimum
path for the packet. The optimality principle defines that if router A is on the optimal path from
router B to router C, then the optimal path from A to C also falls along the same route.
Consequently, the set of optimal routes from all sources to a given destination form a tree
rooted at the destination. Such tree is called a sink tree. The routing table may be either static or

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