What is the purpose of a network mask?

A.
It is used to differentiate a network as a classful network or a classless network.

B.
It is used to hide subnets from external networks by masking the subnets with NAT.

C.
It is used in conjunction with the wildcard mask to determine the number of hosts
on a network.

D.
It is used to define which parts of the IP address are allocated to host addresses
and network prefixes.