What distinguishes the network portion of an IP address from the host portion?

The subnet mask is the key component that distinguishes the network portion of an IP address from the host portion. An IP address is composed of two main parts: the network part, which identifies the specific network, and the host part, which identifies a specific device on that network. The subnet mask works alongside the IP address to define which part of the address refers to the network and which part refers to the host.

When a device needs to communicate on a network, it uses the subnet mask to determine whether the destination address is on the same local network or if it needs to route the request through a gateway. The subnet mask is typically represented in dotted-decimal notation, just like the IP address itself (e.g., 255.255.255.0).

The other options do not serve the same purpose. An IP address identifies a device on a network, a default gateway is the router that connects a local network to other networks, and a DNS address provides the means to resolve domain names to IP addresses. None of these options specifically delineate the network from the host portion of an IP address, which is precisely what the subnet mask is designed to do.