Subnet Calculator

How to use it

1. Type an IPv4 address followed by / and either a prefix (192.168.1.10/24) or a dotted mask (10.0.0.5/255.255.0.0).
2. The result cards fill in: network address, broadcast address, subnet mask, wildcard mask and prefix length.
3. Read the usable host count and the first/last usable IP of the network.
4. The last card shows the address class (A–E) and whether the address is private, loopback, multicast or public.
5. Anything malformed — a bad octet, a prefix over 32, or a non-contiguous mask — shows a clear error instead of a wrong answer.

Common use cases

• Planning VLANs and subnets — checking network/broadcast bounds before assigning ranges.
• Writing firewall or router rules — getting the exact wildcard mask used by ACL syntax.
• Allocating IP ranges — seeing how many usable hosts a /26 or /28 actually gives.
• Studying for network exams — verifying hand-calculated subnet results.

Why it stays private

The whole calculation is integer bit operations inside your browser. There are no lookups and no submitted addresses.

FAQ

Q: Why does a /24 show 254 usable hosts instead of 256? A: The network address itself and the broadcast address cannot be assigned to hosts, so the count is 256 minus 2. The exception is /31 and /32, which follow RFC 3021 point-to-point and single-host conventions.

Q: What masks are rejected? A: Masks whose 1-bits are not contiguous from the left, such as 255.0.255.0. Such masks cannot be expressed as a prefix length and are reported as an error.

Q: What is the wildcard mask for? A: It is the inverse of the subnet mask and is used in many router ACL formats: a wildcard of 0.0.0.255 matches the last octet freely.