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

• A. It separates the network portion from the host portion of an address to identify subnets.
• B. It assigns a unique host ID to each device.
• C. It encrypts the address for security.
• D. It indicates the default gateway.

Answer: (A)

IPv4 addresses are split into a network part and a host part, and the subnet mask marks which bits belong to the network. By applying the mask to an address, you reveal the network address, which tells you which subnet the device is on. This boundary between network and host parts is what lets routers decide where to send traffic: if the destination is on the same subnet, it can be delivered directly; if not, it should be sent to the default gateway on that subnet.

For example, with 192.168.1.50 and a 255.255.255.0 mask, the network is 192.168.1.0, meaning all addresses from 192.168.1.1 to 192.168.1.254 are in the same subnet. The remaining bits identify the specific device within that subnet. The mask doesn’t assign host IDs itself, it doesn’t encrypt addresses, and it doesn’t specify a default gateway—all it does is define which portion of the address is the network portion.