~/tools / Subnet Calculator
Subnet Calculator
Compute CIDR notation, network address, and usable host range from an IP address and subnet mask (e.g. 255.255.255.0).
Router and server config screens often ask for a dotted-decimal subnet mask like "255.255.255.0" — this converts that into CIDR notation (/24, etc.) and the actual host range.
# Use Cases
- Converting a subnet mask shown in a router or OS network settings screen into CIDR notation (/24, etc.)
- Checking the actual range of usable host addresses on a given subnet
- Making sense of a dotted-decimal mask found in an on-prem device's manual
# Usage
- Enter an IP address and subnet mask into their respective fields.
- CIDR notation, network address, and usable host range are calculated automatically as you type.
- An error message appears if the subnet mask is invalid.
- CIDR notation
- 192.168.1.0/24
- Network address
- 192.168.1.0
- Broadcast address
- 192.168.1.255
- First usable host
- 192.168.1.1
- Last usable host
- 192.168.1.254
- Usable hosts
- 254
# FAQ
- How is this different from the CIDR Calculator?
- The CIDR Calculator takes prefix notation like "/24", while this tool takes a dotted-decimal subnet mask like "255.255.255.0". Both produce the same result.
- What makes a subnet mask invalid?
- A subnet mask must be a run of 1 bits followed by a run of 0 bits in binary (e.g. 255.255.254.0 is valid, but 255.255.253.0 is not).