Subnet Mask to CIDR Conversion Table

Subnet Mask to CIDR Conversion

Converting between subnet masks (dotted decimal) and CIDR prefix lengths (slash notation) is a fundamental networking skill. Both representations convey the same information — how many bits define the network portion of an address.

Complete Conversion Table

Here are the most commonly used conversions:

CIDR	Subnet Mask	Usable Hosts
/8	255.0.0.0	16,777,214
/12	255.240.0.0	1,048,574
/16	255.255.0.0	65,534
/20	255.255.240.0	4,094
/21	255.255.248.0	2,046
/22	255.255.252.0	1,022
/23	255.255.254.0	510
/24	255.255.255.0	254
/25	255.255.255.128	126
/26	255.255.255.192	62
/27	255.255.255.224	30
/28	255.255.255.240	14
/29	255.255.255.248	6
/30	255.255.255.252	2
/31	255.255.255.254	2 (P2P link)
/32	255.255.255.255	1

How to Convert Manually

Mask to CIDR: Convert each octet to binary and count the consecutive 1s. For example, 255.255.255.192 = 11111111.11111111.11111111.11000000 = 26 ones = /26.

CIDR to Mask: Write the prefix length as consecutive 1s, pad with 0s to 32 bits, then convert each 8-bit group to decimal. /20 = 11111111.11111111.11110000.00000000 = 255.255.240.0.

Quick Mental Math

For the last octet, memorize these values: 128 (/25), 192 (/26), 224 (/27), 240 (/28), 248 (/29), 252 (/30), 254 (/31), 255 (/32). Each adds one more bit to the network portion.

Usable Hosts Formula

The number of usable hosts is calculated as: 2^(32 - prefix) - 2. The subtraction of 2 accounts for the network address and the broadcast address.