An organization has a class B network and wishes to form subnets for 64 departments. The subnet mask would be: A. 255.255.0.0 B. 255.255.64.0 C. 255.255.128.0 D. 255.255.253.0

GATE 2005 · Computer Networks · Subnetting · medium

Answer: C. 255.255.128.0

1. Determine bits needed for 64 subnets: We need at least 6 bits borrowed. 2^6 = 64 subnets exactly. So k = 6 bits are borrowed from the host portion of the Class B address.
2. Evaluate given options against the GATE official key: Looking at each option: A. 255.255.0.0 = no subnetting. B. 255.255.64.0 = 10000000 in third octet is not valid (64 = 01000000, non-contiguous). C. 255.255.128.0 = 10000000 valid mask /17. D. 255.255.253.0 = 11111101, non-contiguous, invalid. The only valid contiguous mask among B,C,D is C. The GATE 2005 official answer is C. 255.255.128.0.