Subnet Mask Cheat Sheet: CIDR Chart /0 to /32

A subnet mask tells a device which bits of an IPv4 address identify the network. In CIDR notation the number after the slash counts those network bits, so /24 means 255.255.255.0: 256 addresses, 254 of them usable by hosts. The chart below lists every prefix from /32 down to /0 with its dotted mask, wildcard mask and host count.

1. CIDR to Subnet Mask Chart

Usable hosts are the addresses minus the network and broadcast addresses. Two prefixes are exceptions: a /31 gives both addresses to the two ends of a point-to-point link (RFC 3021), and a /32 identifies one host, as in a loopback or host route.

2. Core Formulas

• Host bits = 32 minus the prefix length.
• Addresses per subnet = 2 to the power of the host bits.
• Usable hosts = addresses minus 2 (except /31 and /32).
• Subnets created = 2 to the power of the bits you borrow from the parent network.
• Block size = 256 minus the mask value in the interesting octet.
• Wildcard mask = 255 minus each octet of the subnet mask.

3. Fast Method (do this in your head)

Given an IP address and a mask:

1. Find the interesting octet, the rightmost mask octet that is not 0 or 255 (for /27 it is the fourth octet, 224).
2. Block size = 256 minus that value. For /27, 256 minus 224 = 32.
3. Subnet boundaries are multiples of the block size in that octet: 0, 32, 64, 96, 128, 160, 192, 224.
4. The host's subnet is the largest boundary that is not above the host's value in that octet.
5. Network address = that boundary. Broadcast = the next boundary minus 1.
6. Usable range = network plus 1 up to broadcast minus 1.

4. Worked Examples

Example A: 172.16.45.50/26

• Mask 255.255.255.192, so the block size is 256 minus 192 = 64.
• Boundaries: 0, 64, 128, 192. The value 50 falls in the 0 subnet.
• Network 172.16.45.0, broadcast 172.16.45.63, hosts .1 to .62.

Example B: 10.1.20.130/27

• Mask 255.255.255.224, block size 32.
• Boundaries: 0, 32, 64, 96, 128, 160, 192, 224. The value 130 falls in the 128 subnet.
• Network 10.1.20.128, broadcast 10.1.20.159, hosts .129 to .158.

Example C: 192.168.5.200/29

• Mask 255.255.255.248, block size 8.
• 200 sits exactly on a boundary, so it is the network address of 192.168.5.200/29.
• Broadcast 192.168.5.207, hosts .201 to .206.

Example D: a /30 point-to-point link, 10.10.10.0/30

• Block size 4, so the subnets are .0, .4, .8, .12 and so on.
• For the .0 subnet: network .0, broadcast .3, usable hosts .1 and .2.

Example E: splitting 192.168.1.0/24 into two /25s

• Block size 128.
• 192.168.1.0/25: hosts .1 to .126, broadcast .127.
• 192.168.1.128/25: hosts .129 to .254, broadcast .255.

5. VLSM Design (largest subnet first)

A 200-host LAN, a 60-host LAN, a 28-host LAN and a 2-host WAN link will not fit in one /24, because the 200-host LAN alone needs a /24. Start from 10.0.0.0/23 (510 usable addresses) and allocate the largest block first:

Rule of thumb: sort the networks by size, biggest first, then walk through the address space so every block starts on its own boundary.

6. Wildcard Masks (for ACLs and OSPF)

A wildcard mask is the inverse of the subnet mask. For /27 (mask 255.255.255.224) the wildcard is 0.0.0.31. Cisco ACLs and OSPF network statements use it:

access-list 10 permit 192.168.5.32 0.0.0.31 matches 192.168.5.32 to 192.168.5.63.

7. Reserved IPv4 Ranges

8. Common Mistakes to Avoid

• Confusing the block size with the number of usable hosts: the block includes the network and broadcast addresses.
• Treating a /31 as unusable: under RFC 3021 both addresses work on a point-to-point link.
• Putting a subnet mask where an ACL expects a wildcard mask. They are inverses.
• Counting the broadcast address as a usable host.

Written by CISNET instructors. Print this page (Cmd or Ctrl + P, then Save as PDF) for an offline copy. Work any address with the subnet calculator and build speed with subnetting practice.