CIDR notation looks cryptic — 10.20.0.0/14 — but it describes something simple: a block of consecutive addresses that share the same first bits.
The slash number
An IPv4 address has 32 bits. The number after the slash, the prefix length, says how many of those bits identify the network. The remaining bits identify hosts within it.
/24→ 24 network bits, 8 host bits → 2⁸ = 256 addresses/26→ 26 network bits, 6 host bits → 2⁶ = 64 addresses/30→ 2 host bits → 4 addresses
Every bit you add to the prefix halves the block; every bit you remove doubles it.
Subnet masks are the same thing
Older configuration uses a subnet mask: the prefix written as an address with all network bits set to 1. /24 is 255.255.255.0; /26 is 255.255.255.192. The wildcard mask used in some router ACLs is the inverse: 0.0.0.255 for a /24.
Network, broadcast and usable hosts
Take 192.168.10.77/26:
- The block size is 64, so blocks start at .0, .64, .128 and .192.
- .77 falls in the block starting at .64 → network address
192.168.10.64. - The last address of the block is the broadcast address:
192.168.10.127. - The usable hosts are everything in between:
.65 – .126, which is 62 addresses.
Two special cases: a /31 has no broadcast — both addresses are usable on point-to-point links — and a /32 is a single host.
Splitting a network
To divide 10.0.0.0/22 (1,024 addresses) into four equal subnets, add two bits to the prefix (2² = 4): you get four /24s — 10.0.0.0/24, 10.0.1.0/24, 10.0.2.0/24 and 10.0.3.0/24. Subnets of different sizes are possible too (VLSM), as long as they don’t overlap.
IPv6 is the same, with bigger numbers
IPv6 uses identical notation. The difference is convention: a single LAN is a /64, and a site usually gets a /48 or /56. There is no broadcast address, so every address in a prefix can be used.
Let the calculator do it
The CIDR calculator shows the network, broadcast, masks, usable range and binary breakdown as you type, and the subnet calculator splits any IPv4 or IPv6 network into equal subnets. To turn an arbitrary range into CIDR blocks, use IP range to CIDR.