Please explain clearly why the answer is the one pointed in red router R has the

Question

Please explain clearly why the answer is the one pointed in red

router R has the forwarding table below. (Network addresses have been provided in binary) Network /Mask Next Hop 128.235.223.0/24 (10000000 11101011 11011111 00000000) deliver directly 128.235.176.0/20 (10000000 11101011 10110000 00000000) 128.235.182.1 128.235.172.0/22 (10000000 1110 011100 00000000) 18.235.174.1 0.0.0.0/0 128.235.176.1 What is the next hop for a packet with a destination IP address of 128235.192.10? Destination address in binary is 10000000 11101011 11000000 00001010

Explanation / Answer

When a router receives a packet, it extracts destination IP address. Now it checks if the dentination IP is one of the IP addresses present in the forwarding table, if yes then deliver it to corresponding next hop router otherwise to the default router.

Here if we take each entry one-by-one and compare network part of each entry with destination IP then it will not match with any of these entries and thus forwarded to default router. E.g. first entry in the table is:

10000000 11101011 11011111 00000000 where 24 bits are fixed for network part meaning a packet belonging to this network will have exact same 24 bits values. But given estination IP: 10000000 11101011 11000000 00001010 has only 16 bits matching and thus does not belong to this network. Similarly it does not match with 128.235.176.0/20 and 128.235.172.0/22 and thus correct answer would be the last entry in the table which is for default router.

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