CIDR Cheat Sheet: /8 to /32 Subnet Masks and Host Counts

CIDR Cheat Sheet: Every Prefix From /8 to /32

Bookmark this page. The table below lists every common CIDR prefix from /8 to /32 with its subnet mask, wildcard mask, total addresses, and usable hosts. After the table you will find a quick way to read it and a few rules of thumb that save time in exams and on the job.

If you want the theory behind it, start with our subnetting from scratch guide and come back here as a reference.

The Full CIDR Table

Usable hosts = total addresses minus 2 (the network and broadcast addresses), except for /31 and /32.

How to Read the Table

The number after the slash is how many bits, counted from the left, belong to the network. The remaining bits, 32 - prefix, identify hosts inside it.

• Total addresses = 2^(32 - prefix).
• Usable hosts = total minus 2.
• Every step up in prefix halves the block. Every step down doubles it.

Example: 192.168.10.0/26. Host bits = 6, so 64 addresses, 62 usable. The mask is 255.255.255.192. The block runs from 192.168.10.0 to 192.168.10.63, so the network address is .0 and the broadcast is .63.

Shortcuts Worth Memorising

1. The magic number. In the interesting octet, the block size is 256 - mask value. For /26 the mask octet is 192, and 256 - 192 = 64. Subnets start at 0, 64, 128, 192.
2. Mask octet values. They only ever take these values: 0, 128, 192, 224, 240, 248, 252, 254, 255.
3. Prefix on an octet boundary. /8, /16, /24 line up with 255.0.0.0, 255.255.0.0, 255.255.255.0.
4. Sizing a subnet. Pick the smallest prefix whose usable count covers your hosts. For 50 hosts, /26 (62 usable) fits and /27 (30) does not.

Private Ranges in CIDR Form

These are the RFC 1918 private ranges. For what they mean in practice, see our guide to public vs private IP addresses.

Common Mistakes

• Forgetting to subtract 2. A /24 holds 254 hosts, not 256.
• Mixing up subnet and wildcard masks. Cisco ACLs want 0.0.0.255, not 255.255.255.0.
• Misaligned networks. 10.0.0.100/26 is a host inside 10.0.0.64/26, not a network address. A CIDR block must start on a multiple of its size.
• Overlapping subnets when allocating by hand. Check ranges before assigning.

Skip the Maths When You Need To

For anything beyond a quick lookup, let a tool do it. The IPv4 subnet calculator shows network, broadcast, and host range for any address and prefix. The CIDR range expander lists the addresses in a block, and the wildcard mask calculator converts masks for ACLs. All are free and need no signup.