What is the primary purpose of a subnet mask in relation to an IP address?

The primary purpose of a subnet mask in relation to an IP address is to determine the subnet to which the host belongs. A subnet mask works by separating the network portion of an IP address from the host portion. This allows devices on the same local network (or subnet) to communicate with each other directly and helps routers direct traffic between different networks.

When a device receives an IP address and a subnet mask, it compares the bits in the subnet mask to the bits in the IP address. The subnet mask identifies which part of the IP address corresponds to the network and which part corresponds to the individual device (host). This categorization enables proper routing of packets and efficient use of IP addresses within the network. Without a subnet mask, devices would be unable to determine the correct network segments, leading to routing issues and inefficient communication.

In contrast, the other options relate to different networking concepts. Encrypting the IP address is not the function of a subnet mask. Specifying a default gateway involves directing traffic to external networks, which is not a direct function of the subnet mask itself. Assigning an IP address to the router is about configuring the router, which is a separate task from subnetting.