Subnet Calculator

CIDR in one paragraph

An IPv4 address is 32 bits. CIDR notation writes a prefix length after a slash: /24 means the first 24 bits identify the network and the remaining 8 identify the host. The subnet mask is the same information written as an address, so /24 equals 255.255.255.0. Shorter prefixes mean bigger networks: a /16 holds 65,536 addresses, a /24 holds 256, a /30 holds 4.

Two addresses are not usable

In a standard subnet the first address is the network identifier and the last is the broadcast address, so a /24 with 256 addresses offers 254 usable hosts. This catches people sizing networks: if you need 254 devices you need a /24, and if you need 255 you need a /23. The exception is a /31, which RFC 3021 permits on point-to-point links where both addresses are usable because broadcast is meaningless with exactly two endpoints.

Private ranges

RFC 1918 reserves three blocks for private use: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. The middle one is the one people get wrong, because /12 spans 172.16.x.x through 172.31.x.x, not 172.16 to 172.24. Also reserved: 127.0.0.0/8 for loopback, 169.254.0.0/16 for link-local addresses assigned when DHCP fails, and 100.64.0.0/10 for carrier-grade NAT.

Why subnet

Splitting a network limits broadcast traffic, since a broadcast reaches only its own subnet, and it creates boundaries where access control can be applied. A flat network with thousands of hosts wastes bandwidth on broadcast and gives an attacker who reaches any host direct reachability to all the others.

Plan for growth, but not too much

Renumbering a production network is painful, so leave headroom. The common mistake in the other direction is allocating a /16 to a subnet holding twelve devices, which fragments your address plan and makes route summarisation impossible. Size to expected growth over a few years, not to the largest number you can imagine.