CIDR table: every prefix from /0 to /32

Every IPv4 prefix length with its mask, the number of addresses in the block and how many can be given to devices.

How to read the table

A prefix length such as /24 says how many leading bits of an address name the network. The rest name the device. Each row shows the same block size written three ways: as a prefix, as a subnet mask, and as the count of addresses.

• Addresses is the size of the block: 2 to the power (32 minus the prefix). Each step down the table halves it.
• Usable hosts is two fewer, because the first address names the network and the last is the broadcast address. The exceptions are /31 (a link between two routers, both addresses usable) and /32 (one single address).
• Wildcard mask is the mask inverted. Some firewall rules and routing settings ask for it.

If you came with a number of devices in mind, read up from the bottom and stop at the first row whose "usable hosts" is large enough, then go one row further if the network will grow. If you came with a mask, find it in the second column to learn its prefix. To see the actual range for an address, put it into the subnet calculator.

The sizes you will actually meet

• /32: one address. Used in firewall rules and routing to mean "exactly this device".
• /30 and /31: the link between two routers.
• /29 and /28: a small block of public addresses for a business, 6 or 14 usable.
• /24: the standard home and small office network: 254 devices, mask 255.255.255.0.
• /16: a large site or campus: 65,534 devices. The whole 192.168 private range is one /16.
• /8: the whole 10.x.x.x private range, over 16 million addresses.
• /0: every address there is. In a routing table 0.0.0.0/0 is the "default route", meaning "anything else goes this way".

Splitting and joining blocks

Because each prefix step halves the block, any block splits cleanly into two of the next size: one /24 becomes two /25s, four /26s, eight /27s and so on. The reverse also works when the blocks are neighbours on the right boundary: 192.168.0.0/24 and 192.168.1.0/24 join into 192.168.0.0/23. Networks of different sizes can share one larger block, as long as each starts on a boundary that suits its size. This is how an office divides one range between floors, guests and cameras, and it is the usual subject of subnetting questions in exams.

A quick way to find block boundaries

Subtract the last non-255 number of the mask from 256. The answer is the block size in that position. For 255.255.255.224 (a /27): 256 − 224 = 32, so networks start at 0, 32, 64, 96 and so on. For 255.255.240.0 (a /20): 256 − 240 = 16, so networks start at 0, 16, 32... in the third number. The idea behind this is in subnet masks and CIDR explained.