Subnet

A subnet (subnetwork) is a contiguous block of IP addresses defined by a network prefix, such as 192.168.1.0/24. Hosts in the same subnet talk directly over the local link; traffic to any other subnet goes through a router.

Why it matters

Subnetting keeps broadcast traffic contained, lets you separate departments or server tiers, and makes firewall rules simpler because a whole block can be allowed or denied with one entry. It also lets organizations carve a single allocation into right-sized pieces instead of wasting addresses.

Example

10.0.0.0/16 split into /24 subnets:
10.0.1.0/24  web servers   (10.0.1.1 - 10.0.1.254)
10.0.2.0/24  databases     (10.0.2.1 - 10.0.2.254)

Each /24 holds 256 addresses, of which 254 are usable for hosts. A web server at 10.0.1.5 needs a router to reach a database at 10.0.2.9.

Common mistakes

• Forgetting that the first address (network) and last (broadcast) are not assignable in a typical IPv4 subnet.
• Creating overlapping subnets, which causes routing ambiguity and hard-to-trace outages.
• Making subnets far too small, then running out of addresses when the environment grows.

Related

• CIDR
• Subnet mask
• Router
• VLAN

This page is general information written by our editorial team and updated when documentation changes. Servers and providers differ, so test changes safely first. See our editorial policy and disclaimer.