What is a subnet?
A subnet divides an IP address space into smaller routing groups by specifying network and host portions.
How it works and where it is used
In IPv4, CIDR notation such as /24 describes how many bits form the network prefix. Devices in one prefix may communicate locally depending on design, while routers connect other prefixes. Plan capacity, gateways, growth and overlapping private ranges.
Limits, safety and selection criteria
Subnetting helps organize traffic and administration but is not an access-control rule by itself. Routing and firewall policies enforce separation. IPv6 also uses prefixes, with a very different scale and allocation practice. Giving guest Wi-Fi and accounting systems separate prefixes simplifies address management. If the router freely forwards between them, however, separation offers no access control. A wrong subnet mask can make a device treat remote addresses as local and misroute packets. VLAN, subnet and security zone are related but not identical. Before adding services, check DHCP pools, static reservations and routes advertised upstream. An address plan should be documented so future changes do not create overlapping networks.