Subnet Mask to CIDR Converter - Free Online Tool
Subnet Mask ⇄ CIDR Converter
Convert Between Dotted Mask and CIDR Notation
Understanding Subnet Masks and CIDR
Legacy Config Migration
Older network hardware often expects a dotted-decimal mask instead of modern CIDR notation.
Router Configuration
Quickly translate between formats when a router or firewall's config syntax doesn't match your documentation.
Learning Subnetting
See the direct relationship between a mask's binary pattern and its CIDR prefix length.
Client-Side Only
All calculation happens locally in your browser — no data is ever sent to a server.
Same Concept, Two Notations
A subnet mask and a CIDR prefix express identical information in different forms: the mask 255.255.255.0 has 24 leading 1-bits, exactly matching the CIDR prefix /24. Every valid subnet mask is a contiguous run of 1-bits followed by 0-bits — a mask like 255.255.0.255 (with 0s appearing before 1s) is invalid and this tool will flag it.
Key Takeaways
- Auto-Detected Direction: Enter either a dotted mask or a CIDR prefix — no need to specify which.
- Validates Contiguity: Rejects malformed masks that aren't a valid contiguous run of 1-bits.
- Client-Side Only: All calculation happens locally in your browser — no data is ever sent to a server.