📋 Subnet CIDR Cheat Sheet

A full reference table mapping every CIDR prefix to its subnet mask, wildcard mask and usable host count. Filter by minimum prefix to shorten the table.

Show from /

How each row is built

All three columns come from one 32-bit mask, made by sliding a solid run of 1 bits left until only the host bits are left as zeros:

mask     = 0xFFFFFFFF << (32 - prefix)
wildcard = NOT mask
hosts    = 2^(32 - prefix) - 2    for /0 through /30
         = 2                      for /31  (RFC 3021)
         = 1                      for /32

The /26 row reads 255.255.255.192, wildcard 0.0.0.63, 62 hosts. Mask and wildcard are exact opposites, so each pair of octets adds to 255 - 192 + 63 is the quickest sanity check there is. The /16 row shows 65,534 hosts, because 16 host bits give 65,536 addresses before the network and broadcast come off.

The two prefixes that break the minus-two rule

/31 gives 2, not 0. RFC 3021 allows both addresses of a two-address block to be assigned on a point-to-point link, where there is no shared segment for a broadcast to reach. It halves the addresses a router link costs, from four to two.
/32 gives 1. With no host bits, there is no all-zeros host part to be the network address and no all-ones part to be the broadcast, so nothing is reserved. That is what a loopback interface, a host route or a single-host firewall rule uses.

The "Show from /" box only trims the top of the table. Enter 24 and you get nine rows, /24 down to /32; the /31 and /32 rows are always present, so the sheet never quietly drops the two prefixes that behave differently.

Frequently asked questions

How many hosts are in a /16?

65,534 usable, out of 65,536 addresses. That is the size of a class B network such as 172.16.0.0/16, and the mask is 255.255.0.0.

Can you use a /31 between two routers?

Yes. RFC 3021 defines exactly that case: both addresses in the /31 become the two interface addresses. Most current routers support it, though a few older platforms still refuse to accept the mask.

What does /32 mean in an IP address?

One single address, with the whole 32 bits fixed. It names a specific host rather than a range, which is why access lists, loopbacks and static host routes are written with it.