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How to read the chart
- CIDR is the prefix length: the number of leading one bits in the mask.
- Subnet mask is the same prefix written as four numbers.
- Wildcard is the mask turned inside out.
- Total addresses is every address in the block, 2^(32 - prefix).
- Usable hosts is the total minus 2, because the network and broadcast addresses are not given to devices. A /31 has 2 usable hosts and a /32 has 1.
The bold rows (/8, /16, /24, /30, /31 and /32) are the sizes people meet most often.
Subnet mask and CIDR: the quick way
For /25 to /32 only the last number of the mask changes: /25 is 255.255.255.128, /26 is 255.255.255.192, /27 is 255.255.255.224, /28 is 255.255.255.240, /29 is 255.255.255.248, /30 is 255.255.255.252, /31 is 255.255.255.254, /32 is 255.255.255.255. The last number steps through 128, 192, 224, 240, 248, 252, 254 and 255.
For /17 to /24 the third number steps through the same values, so a /20 is 255.255.240.0 and a /24 is 255.255.255.0. Type any prefix or mask in the converter to see its row.
To turn a mask into a prefix for a whole address, see the CIDR calculator.
Wildcard masks in the chart
Each number in a wildcard mask is 255 minus the matching number in the subnet mask. For a /24 that gives 0.0.0.255, and for a /30 it gives 0.0.0.3.
To go from a wildcard mask back to a prefix, use the wildcard mask calculator.
Print the chart
Press Print chart. The browser prints the full /0 to /32 table on one page, with the navigation, converter, questions and footer hidden. Choose Save as PDF in the print dialog if you want a file.
Frequently asked questions
How many hosts are in a /24?
A /24 has 256 addresses and 254 usable hosts, because the first address is the network address and the last is the broadcast address. Its subnet mask is 255.255.255.0 and its wildcard mask is 0.0.0.255. A /25 has 126 usable hosts, a /26 has 62 and a /27 has 30.
What is the subnet mask for /30?
The subnet mask for /30 is 255.255.255.252, and the wildcard mask is 0.0.0.3. A /30 has 4 addresses and 2 usable hosts, which makes it a common size for a link between two routers. A /31, with mask 255.255.255.254, also gives 2 usable hosts and has no network or broadcast address.
How do I convert a subnet mask to CIDR?
Count the 1 bits in the mask. 255.255.255.0 has 24, so it is /24. 255.255.240.0 has 20, so it is /20. 255.255.255.252 has 30, so it is /30. To go the other way, type the prefix in the converter, such as /20, and it shows 255.255.240.0.
What is the difference between total addresses and usable hosts?
Total addresses is every address in the block, 2^(32 - prefix). Usable hosts is what you can assign to devices. For most prefixes it is the total minus 2, because the network and broadcast addresses are reserved: a /26 has 64 total and 62 usable. Only /31 (2 usable) and /32 (1 usable) differ.
What is a wildcard mask and why is it in the chart?
A wildcard mask is the subnet mask turned inside out: ones become zeros and zeros become ones. The /24 mask 255.255.255.0 becomes 0.0.0.255. Router access lists and routing statements use wildcard masks to say which address bits must match. The chart shows both columns so you do not have to flip bits by hand.
Can I print or save this chart as a PDF?
Yes. Press Print chart and the browser prints the full /0 to /32 table on one page, with the navigation and converter hidden. Choose "Save as PDF" in the print dialog if you want a file. The chart is built in your browser, so it also prints offline after your first visit.