Under classful subnetting, the mask 255.224.0.0 extends a default class mask…

Concept

In classful IPv4 subnetting, the default network prefix is /8 for Class A and /16 for Class B. A subnet mask longer than the default prefix borrows host bits for subnet numbering.

If n host bits are borrowed, counting all subnet-number values gives 2n subnets. Each decimal mask octet can be converted to binary to count its consecutive leading 1-bits.

Application

1. The first mask octet, 255, contributes 8 leading 1-bits. The second octet, 224, is 11100000 in binary and contributes 3 more leading 1-bits, so the mask is /11.

2. Relative to the Class A default /8 prefix, /11 borrows n = 11 − 8 = 3 host bits. A Class B default /16 prefix cannot be extended to the shorter /11 mask by borrowing host bits.

3. The number of subnet-number combinations is 23 = 8.

Cross-check

The three borrowed bits run from 000 through 111, yielding eight distinct combinations. This agrees with the /11 minus /8 calculation.

Therefore, the mask represents Class A with 8 subnets under the stated classful convention.