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CIDR to Subnet Mask Converter

Convert CIDR prefix length (e.g. /24) to a dotted-decimal subnet mask. Includes a full reference table of all CIDR notations.

CIDR Prefix

/

Subnet Mask

255.255.255.0

/24 — 254 usable hosts

CIDR Reference Table

CIDR Subnet Mask Usable Hosts
/0 0.0.0.0 4,294,967,296
/1 128.0.0.0 2,147,483,648
/2 192.0.0.0 1,073,741,822
/3 224.0.0.0 536,870,910
/4 240.0.0.0 268,435,454
/5 248.0.0.0 134,217,726
/6 252.0.0.0 67,108,862
/7 254.0.0.0 33,554,430
/8 255.0.0.0 16,777,214
/9 255.128.0.0 8,388,606
/10 255.192.0.0 4,194,302
/11 255.224.0.0 2,097,150
/12 255.240.0.0 1,048,574
/13 255.248.0.0 524,286
/14 255.252.0.0 262,142
/15 255.254.0.0 131,070
/16 255.255.0.0 65,534
/17 255.255.128.0 32,766
/18 255.255.192.0 16,382
/19 255.255.224.0 8,190
/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 1
/32 255.255.255.255 1

What is CIDR Notation?

CIDR (Classless Inter-Domain Routing) notation is a compact way to specify an IP address and its associated subnet mask. It appends a slash and a number to an IP address — for example, 192.168.1.0/24. The number after the slash is the prefix length, indicating how many of the 32 bits in the address are the network part.

How Conversion Works

The CIDR prefix length tells you how many leading bits of the subnet mask are set to 1. A /24 means the first 24 bits are 1 and the remaining 8 bits are 0, giving a mask of 11111111.11111111.11111111.00000000 = 255.255.255.0.

Common CIDR Prefixes

  • /8 — 255.0.0.0 — Class A default (16M addresses)
  • /16 — 255.255.0.0 — Class B default (65K addresses)
  • /24 — 255.255.255.0 — Class C default (256 addresses)
  • /30 — 255.255.255.252 — Point-to-point links (4 addresses)
  • /32 — 255.255.255.255 — Single host (1 address)