What isolates an IP address's network portion from its host portion?

The correct choice identifies a network mask as the key component that separates an IP address's network portion from its host portion. A network mask, often represented in CIDR notation (e.g., /24) or as a dotted-decimal format (e.g., 255.255.255.0), specifies which part of the IP address refers to the network and which part is used to identify the specific host within that network.

When an IP address is paired with a network mask, the mask defines how to interpret the address—allowing devices to determine whether a destination IP address is within the same local network or if the data needs to be sent to a different network.

In contrast, other concepts listed—such as a gateway, subnet, and router—serve different purposes. A gateway acts as an access point between different networks, a subnet refers generally to a division within a larger network rather than a specification of IP address structure, and routers are devices that forward data packets between networks but do not delineate the segments within an IP address themselves. Thus, a network mask is the precise tool for isolating the network and host portions of an IP address.