What is the function of a subnet mask in an IPv4 address?

The function of a subnet mask in an IPv4 address is to specify the network and host parts of the address. A subnet mask works by determining which portion of the IP address refers to the network itself and which portion identifies the individual device or host within that network.

When an IP address is paired with a subnet mask, the bits in the subnet mask that are set to 1 indicate the network portion, while the bits set to 0 indicate the host portion. For example, if an IP address is 192.168.1.10 and its subnet mask is 255.255.255.0, the mask tells us that the first three octets (192.168.1) identify the network, and the last octet (10) specifies the unique host within that network.

This function is crucial for effective routing of packets on a network, as it allows routers to determine the appropriate destination for data packets by distinguishing between different networks. While the other options relate to aspects of IP addressing and networking, they do not capture the specific role that a subnet mask plays in defining both network and host segments of an IP address.