Subnet Mask Calculator – CIDR to Dotted Decimal

Convert CIDR prefix to dotted-decimal subnet mask and wildcard mask.

CIDR prefix (0-32)

About This Calculator

Convert CIDR prefix to dotted-decimal subnet mask and wildcard mask. Use the calculator above for instant results.

Worked Examples

Example 1: /24

255.255.255.0 | Wildcard: 0.0.0.255

Answer: 255.255.255.0

Example 2: /20

255.255.240.0 | Wildcard: 0.0.15.255

Answer: 255.255.240.0

Example 3: /27

255.255.255.224 | Wildcard: 0.0.0.31

Answer: 255.255.255.224

Who Uses This Calculator?

🔧

Network Engineers

CIDR to dotted-decimal conversion.

📊

IT Admins

Firewall rule configuration.

💻

CCNA Students

Exam prep.

🏫

Networking Students

Quick reference.

Common Mistakes to Avoid

❌ Wildcard vs subnet mask

Subnet: 255.255.255.0. Wildcard: 0.0.0.255. Cisco ACLs use wildcard masks.

❌ Classful is obsolete

Class A/B/C system replaced by CIDR in 1993. Modern networking uses CIDR exclusively.

Frequently Asked Questions

Common masks?

/24=255.255.255.0. /25=255.255.255.128. /26=255.255.255.192. /27=255.255.255.224. /28=255.255.255.240.

Calculate mask from CIDR?

Set first n bits to 1. /24 = 11111111.11111111.11111111.00000000 = 255.255.255.0.

Classful subnets?

Class A: /8. Class B: /16. Class C: /24. Historical — not used for modern routing decisions.