Suppose computers A and B have IP addresses 10.105.1.113 and 10.305.1.91 respectively and they both use the same netmask N. Which of the values given below should not be used if A and B should belong to the same network? A. 255.255.0.0 B. 255.255.128.0 C. 255.255.254.0 D. 255.255.255.0

GATE 2010 · Computer Networks · Subnetting · easy

Answer: D. 255.255.255.0

1. Test mask A: 255.255.0.0 (/16): A: 10.105.1.113 AND 255.255.0.0 = 10.105.0.0 B: 10.105.2.91 AND 255.255.0.0 = 10.105.0.0 Result: SAME network. This mask CAN be used.
2. Test mask B: 255.255.128.0 (/17): Third octet mask = 128 = 10000000 A's third octet: 1 = 00000001; 1 AND 128 = 0 B's third octet: 2 = 00000010; 2 AND 128 = 0 A: 10.105.0.0; B: 10.105.0.0 Result: SAME network. This mask CAN be used.
3. Test mask C: 255.255.254.0 (/23): Third octet mask = 254 = 11111110 A's third octet: 1 = 00000001; 1 AND 254 = 00000000 = 0 B's third octet: 2 = 00000010; 2 AND 254 = 00000010 = 2 A: 10.105.0.0; B: 10.105.2.0 Result: DIFFERENT networks.
4. Test mask D: 255.255.255.0 (/24): Third octet mask = 255 = 11111111 A: 10.105.1.113 AND 255.255.255.0 = 10.105.1.0 B: 10.105.2.91 AND 255.255.255.0 = 10.105.2.0 Result: DIFFERENT networks (10.105.1.0 != 10.105.2.0).