What Is Subnetting? A Plain-Language Guide
Subnetting splits one IP network into smaller networks. It keeps traffic organized, limits broadcast domains, and makes security rules easier to manage.
Why split a network?
A single flat network gets noisy as it grows: every broadcast reaches every device, and one misbehaving machine can affect everyone. Subnetting draws boundaries so each group of devices — a floor, a department, a VLAN — has its own space.
Those boundaries also make firewall and routing rules simpler, because you can allow or block a whole range instead of individual addresses.
Network bits and host bits
An IPv4 address has 32 bits. Part of it identifies the network and the rest identifies a host on that network. Borrowing more bits for the network makes more, smaller subnets.
For example, 192.168.0.0/24 has 256 addresses (254 usable). Split into two /25 networks, each has 128 addresses (126 usable).
Usable hosts
In a normal subnet the first address is the network address and the last is the broadcast address, so usable hosts = total addresses − 2. A /27 has 32 addresses and 30 usable hosts. Point-to-point /31 links and /32 host routes are the exceptions.