CIDR Notation Chart: Full /0 to /32 Cheat Sheet

A /26 prefix means the first 26 bits of the IPv4 address are the network portion, which gives a subnet mask of 255.255.255.192 and a wildcard mask of 0.0.0.63. That leaves 6 host bits, so a /26 block contains 2 to the power of 6 = 64 total addresses, of which 62 are usable for hosts after subtracting the network and broadcast addresses. A /26 is exactly one quarter of a /24, so a single 255.255.255.0 network divides cleanly into four /26 subnets: .0–.63, .64–.127, .128–.191 and .192–.255. It is a popular size for small office segments or VLANs that need between 31 and 62 devices, where a /24 would waste addresses and a /27 (30 hosts) would be too small.

CIDR notation is written as an IP address, a forward slash, and a prefix length — for example 192.168.1.0/24 for IPv4 or 2001:db8::/48 for IPv6. The number after the slash is the count of leading bits that are fixed as the network portion of the address. IPv4 prefixes range from /0 to /32 because an IPv4 address is 32 bits long; IPv6 prefixes range from /0 to /128. The format replaced the older class-based system (Class A, B, C) so a network can be any size rather than a fixed boundary. Reading it is direct: the prefix tells you how many network bits there are, and 32 minus that prefix (for IPv4) tells you how many host bits remain to size the block.

For ordinary IPv4 subnets you take 2 to the power of the host bits, then subtract 2 for the network and broadcast addresses. A /24 has 8 host bits, so 256 total addresses and 254 usable. A /26 has 6 host bits, so 64 total and 62 usable. A /30 has 2 host bits, so 4 total and 2 usable — the classic size for a point-to-point link between two routers. The two exceptions to the minus-2 rule are at the very end of the range: a /31 (RFC 3021) is used for point-to-point links and treats both of its 2 addresses as usable, and a /32 describes a single host route with exactly one address. The full chart on this page lists every prefix from /0 to /32 so you never have to compute the powers of two by hand.

A subnet mask and a wildcard mask are bitwise inverses of each other. The subnet mask marks the network bits with ones and the host bits with zeros — for a /24 that is 255.255.255.0. The wildcard mask flips every bit, marking the host bits with ones, so the /24 wildcard is 0.0.0.255. Subnet masks are what you configure on interfaces and see in routing tables, while wildcard masks appear in access control lists and OSPF network statements, especially on Cisco equipment, where they describe which bits to match and which to ignore. They always sum to 255 in each octet, so converting between them is just 255 minus each octet. This chart lists both forms side by side for every prefix.