What is the purpose of a subnet mask in IPv4 addressing?

• A. It assigns the default gateway for the network.
• B. It translates hostnames to IP addresses.
• C. It determines the broadcast address for an entire network.
• D. It defines which bits are network versus host, enabling subnetting and address separation.

Answer: (D)

Subnet masks define which bits in an IPv4 address belong to the network portion and which belong to the host portion. They are written as 32-bit values with 1s indicating network bits and 0s indicating host bits. This separation lets you subdivide an address space into subnets (subnetting) and determine where to route traffic within a network. When you apply the mask to an IP address, you can derive the network address, which identifies the specific subnet, and you can infer the range of usable host addresses in that subnet. The broadcast address can be derived by setting all host bits to 1, but the essential purpose of the mask is to distinguish network versus host bits for proper addressing and subnetting. The other options describe different network functions (gateway assignment, name resolution) that are not what the mask determines.