Networks: Classless Inter Domain Routing

Question

Can anyone help me out in my revision of networks by helping me understand this question and how to go about answering it?

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A router supporting variable length subnet masks and classless inter-domain routing (CIDR) has the following forwarding entries:

Showing full details of your working identify: Which next hop entries would match each of the following destination IP addresses, and in each case, which next hop would the router select?

1. 10.20.96.16

2. 10.20.130.16

3. 192.168.87.4

4. 192.168.89.2

Answer 1

It is simple.The mask corresponding to the entries are as follows:

0.0.0.0/0 has mask 00000000 00000000 00000000 00000000

10.20.0.0/17 has mask 11111111 11111111 10000000 00000000

10.20.64.0/18 has mask 11111111 11111111 11000000 00000000

192.168.80.0/21 has mask 11111111 11111111 11111000 00000000

192.168.88.0/22 has mask 11111111 11111111 11111100 00000000

Considering first address 10.20.96.16

In binary : 00001010 00010100 01100000 00010000

It is ANDed with the masks of various entries and result is compared with base address of corresponding entry, looking for a match.The entries that match are:

A,B,C

For example after performing AND operation with mask of 2nd entry you get

0000 1010 00010100 00000000 00000000 and this matches base address 10.20.0.0

But router selects the longest prefix match, so C is selected as next hop.

Considering second address 10.20.130.16, performing similar procedure,the next hop entries that match are:

A

and since only one match the next hop selected by router is A

For third address 192.168.87.4, performing similar procedure,the next hop entries that match

A,D

and the router selects the next hop as D(longest prefix match)

For fourth address 192.168.89.2,performing similar procedure,the next hop entries that match

A,E

and router selects E as the next hop.