Question

in the longest prefix matching rule, if I have an address that matches 2 interfaces, which interface should I choose ?

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is the router the same as interface ? and what is the default address for a router ?

Thank you

Answer 1

If a incoming packet maps to more than one entry in routing table,the packet is forwarded to interface having longest prefix .

Example:

Prefix next hop

131.16.0.0/12 3

131.28.0.0/14 5

131.22.0.0/15 1

if router recieves a packet with address 131.23.151.76,following is done:

131.16.0.0/12 =>subnet mask=255.240.0.0

131.23.151.76 bitwise and 255.240.0.0=131.16.0.0

Hence the network address of packet and prefix are matching.

131.28.0.0/14=>subnet mask=255.252.0.0

131.23.151.76 bitwise and 255.252.0.0=131.20.0.0

Here network address of packet and prefix are not matching.

131.22.0.0/15=>subnet mask=255.254.0.0

131.23.151.76 bitwise and 255.254.0.0=131.22.0.0

Hence the network address of packet and prefix are matching

Here prefixes of 3 and 1 are matching.

But according to longest prefix matching rule, we have to forward it to 131.22.0.0/15 instead of  131.16.0.0/12 .(since 131.22.0.0/15 has longer prefix than 131.16.0.0/12)

Router is not same as interface.

A router has more than one interface and each interface is assigned a ip address.

default address of a router is the address of interface on which a packet is forwarded when packet doesn't match to any of entries in router.

Generally default address =0.0.0.0 and subnet mask is 0.0.0.0.

Hence if a packet doesn't match with any of entries and ip address bitwise and 0.0.0.0=0.0.0.0 which is default address.

Hence packet is forwarded onto that interface.