What is the purpose of a network mask?（）

• A、It is used in conjunction with the wildcard mask to determine the number of hosts on a network
• B、It is used to define which parts of the IP address are allocated to host addresses and network prefixes
• C、It is used to hide subnets from external networks by masking the subnets with NAT
• D、It is used to differentiate a network as a classful network or a classless network