What is a subnet mask?

A subnet mask is a crucial component in IP networking as it serves the primary function of dividing an IP address into two key parts: the network part and the host part. This division helps in identifying which portion of the address is used to designate the network and which part is used for host addresses within that network.

Subnet masks are typically represented in dotted-decimal notation, similar to IP addresses (for example, 255.255.255.0). By applying the subnet mask to an IP address, a network device can determine the network's size and find out which devices belong to the same network segment. This is critical for routing traffic between different networks and ensuring efficient use of the available IP address space.

The other options described do not accurately define a subnet mask. For instance, a protocol for network communication refers to standards and procedures that allow devices on a network to communicate, which is distinct from the function of a subnet mask. Similarly, a tool for network speed testing would measure how fast data can travel across a network rather than defining how addresses are structured within that network. Lastly, while the address format for networks includes subnet masks, it is a broader category encompassing many types, including IP addresses and other addressing schemes, which does not specifically