GATE | GATE-CS-2003 | Question 90

The subnet mask for a particular network is 255.255.31.0. Which of the following pairs of IP addresses could belong to this network?
(A) 172.57.88.62 and 172.56.87.233
(B) 10.35.28.2 and 10.35.29.4
(C) 191.203.31.87 and 191.234.31.88
(D) 128.8.129.43 and 128.8.161.55


Answer: (D)

Explanation:
Background required – Subnetting and SubNet Mask

Answer – D

Explanation – Suppose we have a host A with IP Address IP_A and Subnet mask of the subnet of which A is a part is M_s. Now when A wants to send packet to a host B with an IP address say IP_B, then A will first AND the subnet-Mask M_s with IP_A to find out the subnet-id ID_N of the subnet in order to identify whether IP_B belongs to the same network. Now if B also belongs to the same network, then IP_B BITWISE-AND M_s must be equal to ID_N which A has calculated.

If IP_A and IP_B belongs to same network, we must have

    IPA BITWISE-AND Ms == IPBBITWISE-AND Ms

Out of the 4 options, only option D matches.

128.8.129.43 BITWISE-AND 255.255.31.0 = 128.8.1.0
128.8.161.55 BITWISE-AND 255.255.31.0 = 128.8.1.0

You can check rest of the options.

This explanation has been provided by Pranjul Ahuja.

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