An IPv4 address has first octet 100. A classful network requires at least 80…

Concept

In fixed-length classful subnetting, borrowing s bits from the host portion creates 2s subnet identifiers. To maximize hosts while meeting a requirement of N subnets, choose the smallest s for which 2s is at least N. In a mask octet, s borrowed bits appear as s leading 1-bits followed by 0-bits.

Application

The first octet 100 lies in the Class A range 1–126, so the classful default prefix is /8. Apply the minimum-bits rule:

1. With 6 borrowed bits, 26 = 64, which is fewer than 80.

2. With 7 borrowed bits, 27 = 128, which is at least 80; therefore 7 is the minimum.

3. The new prefix is /8 + 7 = /15. Seven leading 1-bits in the second mask octet give 11111110 in binary, or 254 in decimal, so the full mask is 255.254.0.0.

Cross-check

A Class A subnet table lists 7 borrowed bits as 255.254.0.0 (/15), providing 128 subnet identifiers and 131070 usable host addresses per subnet. One fewer borrowed bit gives 255.252.0.0 and only 64 subnet identifiers; borrowing more bits would reduce the host capacity unnecessarily.

Result

The required second-octet value is 254, giving subnet mask 255.254.0.0 (/15).