CIDR and subnet masks explained

In firewall rules, server settings or your home router you meet notations like 192.168.1.0/24. This is CIDR, a compact way to describe an IP range.

What does /24 mean?

An IPv4 address has 32 bits. The number after the slash says how many of them define the network. In a /24 the first 24 bits are fixed and the remaining 8 are for devices, giving 256 addresses. A /26 leaves 6 bits, so 64 addresses. Each extra prefix bit halves the block.

Why two fewer usable addresses?

The first address of a block is the network and the last is the broadcast address; neither is given to devices. A /24 therefore has 254 usable addresses and a /26 has 62. Two-address /31 links are the exception.

Subnet masks

255.255.255.0 is the older notation for /24: the 255 parts are the network, the 0 parts are devices. Some firewalls want the inverse, the wildcard mask (0.0.0.255).

Private ranges

10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 are used only on local networks and are not routed on the internet. If a website's DNS record shows one of these, the site cannot be reached from outside.

The IP subnet calculator works out every address of a block and can split it into smaller subnets.