IP Routing

Classful Route Lookups

Router branch1 is configured to perform classful route lookups (see line 7 in the previous code block). A classful route lookup works as follows:

1. Upon receiving a packet, the router first determines the major network number for the destination. If the destination IP address is 172.16.1.1, the major network number is 172.16.0.0. If the destination IP address is 192.168.1.1, the major network number is 192.168.1.0.

2. Next, the router checks to see if this major network number exists in the routing table. If the major network number exists in the routing table (172.16.0.0 does), the router checks for the destination’s subnet. In our example, branch1 would look for the subnet 172.16.1.0. If this subnet exists in the table, the packet will be forwarded to the next hop specified in the table. If the subnet does not exist in the table, the packet will be dropped.

3. If the major network number does not exist in the routing table, the router looks for a default route. If a default route exists, the packet will be forwarded as specified by the default route. If there is no default route in the routing table, the packet will be dropped.

Router branch1 is able to ping 192.168.1.1 as a consequence of rule 3:

branch1#ping 192.168.1.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 40/50/80 ms

However, let’s define a new subnet of 172.16.0.0 on core1 (and then block ...