An IP address is really two pieces of information wearing one coat: which network you are on, and which host you are on that network. The subnet mask is what says where the line between the two falls. A mask of 255.255.255.0 puts that line after 24 bits, leaving the last part of the address to identify the host.
That line decides behaviour. When a device wants to send something, it compares its own address and the destination address against the mask. If both fall on the same network, it delivers directly across the local link. If they do not, it hands the packet to its default gateway and lets a router deal with it. Splitting address space into subnets is how large blocks get divided into usable pieces.
Three things to remember
- The mask marks where the network part of an address ends and the host part begins.
- A device uses it to choose between local delivery and handing the packet to the gateway.
- The all-zeros and all-ones host values in a subnet are reserved, which is why a /24 offers 254 usable addresses rather than 256.
Related explanations
- What Happens When Your Whole House Is Online at Once? (The Internet Only Sees One of You)
- Why Won’t These Two Computers Talk? (For Once, It’s Not DNS)
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