IP Address Today

Subnet mask cheat sheet

The whole IPv4 prefix table in one place, generated from the same code that powers the subnet calculator. Convert a mask you already have using the box below, or read the table straight off.

Try:
CIDR prefix
/26
Subnet mask
255.255.255.192
Wildcard mask(ACLs)
0.0.0.63
Total addresses
64
Usable hosts
62
Relative size
one 4th of a /24

What the prefix actually means

The number after the slash is how many leading bits of the address identify the network. Everything left over identifies the host. A /24 fixes the first 24 bits, leaving 8 bits — 256 addresses — for hosts.

Each step shorter doubles the block. That is worth internalising, because it makes the whole table something you can reconstruct rather than look up: a /23 is two /24s, a /22 is four, a /16 is 256.

The full table

IPv4 CIDR prefix, subnet mask, wildcard mask, address count and usable host count
PrefixSubnet maskWildcardAddressesUsable hosts
/32255.255.255.2550.0.0.011
/31255.255.255.2540.0.0.122
/30255.255.255.2520.0.0.342
/29255.255.255.2480.0.0.786
/28255.255.255.2400.0.0.151614
/27255.255.255.2240.0.0.313230
/26255.255.255.1920.0.0.636462
/25255.255.255.1280.0.0.127128126
/24255.255.255.00.0.0.255256254
/23255.255.254.00.0.1.255512510
/22255.255.252.00.0.3.2551,0241,022
/21255.255.248.00.0.7.2552,0482,046
/20255.255.240.00.0.15.2554,0964,094
/19255.255.224.00.0.31.2558,1928,190
/18255.255.192.00.0.63.25516,38416,382
/17255.255.128.00.0.127.25532,76832,766
/16255.255.0.00.0.255.25565,53665,534
/15255.254.0.00.1.255.255131,072131,070
/14255.252.0.00.3.255.255262,144262,142
/13255.248.0.00.7.255.255524,288524,286
/12255.240.0.00.15.255.2551,048,5761,048,574
/11255.224.0.00.31.255.2552,097,1522,097,150
/10255.192.0.00.63.255.2554,194,3044,194,302
/9255.128.0.00.127.255.2558,388,6088,388,606
/8255.0.0.00.255.255.25516,777,21616,777,214
/7254.0.0.01.255.255.25533,554,43233,554,430
/6252.0.0.03.255.255.25567,108,86467,108,862
/5248.0.0.07.255.255.255134,217,728134,217,726
/4240.0.0.015.255.255.255268,435,456268,435,454
/3224.0.0.031.255.255.255536,870,912536,870,910
/2192.0.0.063.255.255.2551,073,741,8241,073,741,822
/1128.0.0.0127.255.255.2552,147,483,6482,147,483,646
/00.0.0.0255.255.255.2554,294,967,2964,294,967,294

Frequently asked

Why does a /24 have 254 usable hosts and not 256?
The first address in a block is the network address and the last is the broadcast address, and neither can be assigned to a host. That leaves 254 of the 256. The exceptions are /31, where RFC 3021 makes both addresses usable on point-to-point links, and /32, which is a single host route.
What is a wildcard mask for?
It is the bitwise inverse of the subnet mask, and Cisco ACLs and OSPF use it where other tools use the mask. 0.0.0.255 in a wildcard mask means the same set of addresses as 255.255.255.0 in a subnet mask — the bits set to zero are the ones that must match.
Are subnet masks always contiguous?
In every modern system, yes: a valid mask is some number of 1 bits followed by 1 bits only to their left, which is what lets it be written as a single prefix length. Non-contiguous masks were legal in early specifications and are now rejected — which is why 255.255.0.255 is not a mask this or any current tool will accept.
How do I pick a prefix for N hosts?
Find the smallest block whose usable host count is at least N, remembering the two reserved addresses. For 100 hosts you need a /25 (126 usable), not a /26 (62). Leave room: renumbering a subnet that has run out is far more work than allocating one size larger at the start.

Doing this for thousands of addresses?

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curl https://api.ipaddresstoday.com/api/v1/8.8.8.8
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