IPv4 Subnet Calculator — CIDR, Mask, Host Range

How CIDR notation works

An IPv4 address is 32 bits. The prefix length after the slash says how many of those bits identify the network; the rest identify the host. In 192.168.1.10/24, the first 24 bits are the network (192.168.1) and the last 8 are the host (10).

Everything else follows from that split:

• Subnet mask — the prefix bits set to 1. A /24 is 255.255.255.0.
• Network address — the address with all host bits set to 0. This identifies the subnet itself and is not assignable.
• Broadcast address — all host bits set to 1. Traffic sent here reaches every host on the segment, so it is not assignable either.
• Wildcard mask — the inverse of the subnet mask. Cisco ACLs and OSPF network statements use this form rather than the mask.

Why usable hosts is total minus two

A /24 contains 2⁸ = 256 addresses, but only 254 can be assigned to hosts, because the network and broadcast addresses are reserved. The general formula is:

Usable hosts = 2^(32 − prefix) − 2

This is the single most common source of off-by-two errors in address planning. A /30, often used for point-to-point links, has four addresses and exactly two usable — which is why it was the traditional choice for a router-to-router link.

The /31 and /32 exceptions

Two prefixes break the rule, and this calculator handles them explicitly:

• /31 — RFC 3021 allows both addresses to be used on a point-to-point link, because a broadcast address is meaningless when there are exactly two endpoints. This halves the address waste compared with a /30, and modern practice prefers it.
• /32 — a single host address, used for loopback interfaces and host routes. There is no network or broadcast to reserve.

Reference table

Private address ranges

RFC 1918 reserves three blocks that are never routed on the public internet:

• 10.0.0.0/8 — 16.7 million addresses, the usual choice for large internal networks
• 172.16.0.0/12 — 1 million addresses; note the range ends at 172.31.255.255, not 172.255.x.x
• 192.168.0.0/16 — 65,536 addresses, ubiquitous in small networks and consumer equipment

Also worth recognising: 169.254.0.0/16 is link-local (APIPA), and an interface holding one of those addresses has usually failed to reach DHCP — a useful diagnostic in itself.

A note on subnetting practice

Sizing a subnet exactly to today's host count is a false economy. Renumbering a live segment is disruptive, and address space inside RFC 1918 is effectively free. Leave room, and keep boundaries on easily-readable bit positions — /24 rather than /23 or /25 — unless you have a concrete reason. Whoever debugs the network at 3 a.m. will thank you.