CIDR cheat sheet (common subnet sizes)

This cheat sheet gives you the most commonly used CIDR ranges along with their subnet masks, block sizes, and usable host counts.

It is designed for quick reference when working with firewalls, cloud networking, VLANs, or troubleshooting IP ranges.

CIDR   Subnet Mask         Total IPs   Usable Hosts   Block Size

/30    255.255.255.252     4           2              4
/29    255.255.255.248     8           6              8
/28    255.255.255.240     16          14             16
/27    255.255.255.224     32          30             32
/26    255.255.255.192     64          62             64
/25    255.255.255.128     128         126            128
/24    255.255.255.0       256         254            256

CIDR   Subnet Mask         Total IPs   Usable Hosts

/23    255.255.254.0       512         510
/22    255.255.252.0       1024        1022
/21    255.255.248.0       2048        2046
/20    255.255.240.0       4096        4094
/16    255.255.0.0         65536       65534

Block sizes double as the subnet gets larger. Each step up halves the number of networks and doubles the number of hosts.

/30 → 4
/29 → 8
/28 → 16
/27 → 32
/26 → 64
/25 → 128
/24 → 256

• /24 is the most common subnet (256 addresses)
• /27 and /28 are common for smaller segments
• /30 is often used for point-to-point links
• Always subtract 2 addresses for usable hosts (network and broadcast)

• /30 → point-to-point links
• /29 → very small networks
• /27 → small office segments
• /24 → standard LAN or VLAN
• /16 → large internal networks

Instead of memorising everything, you can instantly calculate subnet ranges using DNS Pro: https://app.dnspro.co.uk

Enter an IP address with CIDR or subnet mask to get the full range, broadcast address, and usable hosts.

Learn the fundamentals: /articles/what-is-cidr-notation

Understand subnet masks: /articles/what-is-a-subnet-mask

Step-by-step calculations: /articles/how-to-calculate-a-subnet-range