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Question

An IP router implementing CIDR services a packet with address 131.23.151.76.The routers routing table has the following entries

Prefix.            Output interface identifier

131.16.0.0/12                          3

131.16.0.0/14                           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,.,........

Comments

output interface 1 ?

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Yes but how???

Answer 1

Start with the longest prefix - /16
Subnet mask for it - 255.255.0.0

Subnet mask AND IP = Network ID

131.23.151.76 AND  255.255.0.0 = 131.23.0.0/16 (Not matched with 131.19.0.0/16 )

Next is /15
Subnet mask for it - 255.254.0.0
131.23.151.76 AND  255.254.0.0 = 131.22.0.0/15  (Matched)

So The identifier of the output interface on which this packet will be forwarded is 1 .