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

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