Question

1. Consider a datagram network using 32-bit host addresses. Suppose a router has three links, numbered 0 through 2, and packets are to be forwarded to the link interfaces as follows:

Prefix Link Interface

11100000 00 0

11100000 01000000 1

Other 2

Determine output interface for each of the following three destination addressed:

11001000 10010001 01010001 01010101

11100000 00101010 00000000 10101010

11100000 01000000 10000001 01000010

Answer 1

1. This address does not match with either first prefix of second prefix, hence output interface will be 2 [Other - 2 in forwarding rule].
2. This address matches with prefix 1 (11100000 00), hence output interface will be 0.
3. This address matches with prefix 2 (11100000 01000000), hence output interface will be 1.

For determining output interface we use longest prefix matching, where the output interface is determined using the prefix with longest match. If two or more prefixes match, then we take that prefix with longer length.