Subnet Cheat Sheet

Every IPv4 prefix with its subnet mask, wildcard mask and host count. Click a prefix for details, or open thesubnet calculator.

CIDR to subnet mask table

CIDR to subnet mask table
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
/00.0.0.0255.255.255.2554,294,967,296— (default route)

*A /31 has no network or broadcast address when used on a point-to-point link (RFC 3021), so both addresses are usable.

Block size (magic number) table

Block size by mask octet value
Mask octet value128192224240248252254255
Block size1286432168421
Prefix (4th octet)/25/26/27/28/29/30/31/32
Prefix (3rd octet)/17/18/19/20/21/22/23/24

How to subnet in your head

  1. Find the "interesting" octet: the one where the mask is not 255 or 0.
  2. Block size = 256 − mask value in that octet. For 255.255.255.192, 256 − 192 = 64.
  3. Networks start at multiples of the block size: .0, .64, .128, .192.
  4. The broadcast is one less than the next network. For 10.1.1.70/26, the network is 10.1.1.64 and the broadcast is 10.1.1.127; usable hosts are .65 – .126.

Private and special IPv4 ranges

Private and special IPv4 ranges
RangePurpose
10.0.0.0/8Private (RFC 1918)
172.16.0.0/12Private (RFC 1918)
192.168.0.0/16Private (RFC 1918)
100.64.0.0/10Carrier-grade NAT (RFC 6598)
127.0.0.0/8Loopback
169.254.0.0/16Link-local (APIPA)
224.0.0.0/4Multicast

Frequently asked questions

How do I memorize subnet masks?

Learn the eight values one octet can take (128, 192, 224, 240, 248, 252, 254, 255). Each adds one bit. The block size is 256 minus that value.

What is the formula for usable hosts?

2ⁿ − 2, where n = 32 − prefix length. For /27, n = 5, so 32 − 2 = 30.

How many subnets do I get by borrowing bits?

2ᵇ, where b is the number of bits borrowed. Turning a /24 into /27s borrows 3 bits and creates 8 subnets.

What's the difference between a subnet mask and a wildcard mask?

The wildcard is the inverse: subtract each mask octet from 255. Cisco ACLs and OSPF use wildcards.

Can I print this cheat sheet?

Yes. Use the Print button; the table fits on one page.