Question

An Ip router implementing Classless Inter-domain Roting (CIDR) receives a packet with address 131.23.151.76. the router’s routing table has the following entries:

Prefix	Output Interface Identifier
131.16.0.0/12	3
131.28.0.0/14	5
131.19.0.0/16	2
131.22.0.0/15	1

 

The identifier of the output interface on which this packet will be forwarded is ________

Answer 1

Explanation:

First field of routing table: 131.16.0.0/12

Given mask bits = 12

131.23.151.76 = 131.00010111.151.76

131.00010111.151.76

& 131.00010000.0.0

131.16.0.0

 

BITWISE AND Result (Matched with 131.16.0.0)

Second field of routing table: 131.28.0.0/14

Given mask bits = 14

131.00010111.151.76

& 131.00010100.0.0

131.20.0.0

 

BITWISE AND Result (Not Matched with 131.28.0.0)

Third field of routing table: 131.19.0.0/16

Given mask bits = 16

131.00010111.151.76

& 131.00010111.0.0

131.23.0.0

 

BITWISE AND Result (Not Matched with 131.19.0.0)

Fourth field of routing table: 131.22.0.0/15

Given mask bits = 15

131.00010111.151.76

& 131.00010110.0.0

131.22.0.0

 

BITWISE AND Result (Matched with 131.22.0.0)

Here 1^st and 4^th both fields are matched but we will take the field with higher mask bits so interface 1 is the correct answer.