How do you convert a subnet mask to CIDR notation?

Count the number of 1 bits in the subnet mask. Convert the mask to binary and count the leading 1s. For example, 255.255.255.0 in binary is 11111111.11111111.11111111.00000000 — that is 24 ones, so the CIDR prefix is /24. For 255.255.240.0: 11111111.11111111.11110000.00000000 — 20 ones, so /20.

How do you convert CIDR notation to a subnet mask?

Write a 32-bit number with the first n bits set to 1 and the rest to 0, where n is the CIDR prefix length. Then convert to dotted-decimal. For /26: 11111111.11111111.11111111.11000000 = 255.255.255.192. For /20: 11111111.11111111.11110000.00000000 = 255.255.240.0.

What is a wildcard mask?

A wildcard mask (inverse mask) is the bitwise NOT of the subnet mask. Where the subnet mask has 1s (network bits), the wildcard has 0s, and vice versa. For /24 (subnet mask 255.255.255.0), the wildcard is 0.0.0.255. Wildcard masks are used in Cisco ACLs and OSPF area statements.

Fundamentals

Subnet Mask vs. CIDR Notation

How to convert between subnet masks (255.255.255.0) and CIDR prefix lengths (/24) — a complete conversion table and the math behind it.

Two ways to say the same thing

A subnet mask and a CIDR prefix length convey identical information — just in different formats.

CIDR Notation

/24

Subnet Mask

255.255.255.0

Binary

11111111.11111111
11111111.00000000

A subnet mask uses dotted-decimal notation where each 255 in an octet means all 8 bits are set to 1 (network bits), and 0 means all host bits. The CIDR prefix just counts those 1 bits: /24 means 24 bits are set — 255.255.255.0.

Complete conversion table /0–/32

CIDR	Subnet Mask	Wildcard	Usable Hosts	Common Use
/0	0.0.0.0	255.255.255.255	4,294,967,294	Default route
/8	255.0.0.0	0.255.255.255	16,777,214	Class A / 10.x.x.x
/12	255.240.0.0	0.15.255.255	1,048,574	172.16.0.0/12
/16	255.255.0.0	0.0.255.255	65,534	VPC / campus
/17	255.255.128.0	0.0.127.255	32,766	Half /16
/18	255.255.192.0	0.0.63.255	16,382	Quarter /16
/19	255.255.224.0	0.0.31.255	8,190	Large segment
/20	255.255.240.0	0.0.15.255	4,094	AWS default subnet
/21	255.255.248.0	0.0.7.255	2,046	Building network
/22	255.255.252.0	0.0.3.255	1,022	Medium segment
/23	255.255.254.0	0.0.1.255	510	Two /24s
/24	255.255.255.0	0.0.0.255	254	Standard subnet
/25	255.255.255.128	0.0.0.127	126	Half /24
/26	255.255.255.192	0.0.0.63	62	Quarter /24
/27	255.255.255.224	0.0.0.31	30	Small workgroup
/28	255.255.255.240	0.0.0.15	14	AWS NAT/endpoint
/29	255.255.255.248	0.0.0.7	6	Tiny cluster
/30	255.255.255.252	0.0.0.3	2	P2P link
/31	255.255.255.254	0.0.0.1	2	RFC 3021 P2P
/32	255.255.255.255	0.0.0.0	1	Single host

How to convert between them

CIDR → Subnet Mask

Count /prefix ones from the left in a 32-bit number:

For /26: 26 ones then 6 zeros = 11111111.11111111.11111111.11000000 = 255.255.255.192

Subnet Mask → CIDR

Count the 1 bits in the mask. 255.255.240.0 = 11111111.11111111.11110000.00000000 = 20 ones = /20