CIDR Range Calculator

What the slash actually says

CIDR notation writes an address followed by a prefix length: 10.0.0.0/24. The number is how many leading bits are fixed as the network portion. Everything after them varies, so the block contains 2 to the power of the remaining bits. A /24 fixes 24 of 32 bits, leaving 8, giving 256 addresses. A /16 leaves 16 bits and 65,536. Each step down the prefix doubles the block.

The base address must be aligned

10.0.1.0/24 is valid; 10.0.1.5/24 is not a block, it is a host inside one. A CIDR block must start on a multiple of its own size, because the host bits are all zero in the network address. Tools differ on whether they reject a misaligned input or silently normalise it to the containing block, which is a real source of configuration bugs when a firewall rule quietly covers 256 addresses instead of the one intended.

Usable hosts versus total addresses

A /24 holds 256 addresses but 254 usable hosts, because the first is the network address and the last is the broadcast. This applies to IPv4 subnets on a shared segment. It does not apply to a /32, which is a single host route, nor to /31, which RFC 3021 defines as a two-address point-to-point link with no broadcast. Cloud providers reserve further addresses inside each subnet — AWS takes five per subnet, which surprises people sizing a /28.

Checking for overlap

Two blocks overlap if one contains the other's network address. The practical test is to compare them at the shorter of the two prefix lengths: if the masked addresses match, they overlap. Overlapping ranges in routing tables, security groups or VPN configurations produce behaviour that depends on rule ordering and longest-prefix matching, which is far harder to debug than an outright conflict.

Supernetting and route summarisation

Contiguous blocks that share a prefix can be advertised as one shorter prefix: 10.0.0.0/24 and 10.0.1.0/24 combine into 10.0.0.0/23. This only works when the blocks are adjacent and the combined block is properly aligned — 10.0.1.0/24 and 10.0.2.0/24 cannot be summarised into a /23, despite being consecutive, because /23 boundaries fall on even third octets.

IPv6 uses the same notation, different scale

A /64 is the standard IPv6 subnet and contains more addresses than the entire IPv4 space squared. Prefix arithmetic works identically; the intuitions about conserving addresses do not, and applying IPv4 frugality to IPv6 allocation causes more problems than it solves.