What does a subnet calculator do?
Every IP network is defined by an address and a prefix length written in CIDR notation, such as
192.168.1.0/24. The prefix says how many leading bits identify the network; the remaining bits number the
hosts inside it. Working out the netmask, broadcast address and usable range by hand means converting to binary and
counting bits – exactly the kind of task where mistakes slip into firewall rules and VPC designs. This calculator does it
instantly for IPv4 and IPv6, using Python's standard ipaddress module on our server.
What you get
- Network address, netmask, wildcard mask (for Cisco ACLs and OSPF) and broadcast address.
- First and last usable host and the number of usable hosts, with the special rules for
/31point-to-point links (RFC 3021) and/32host routes. - The address type: public, private (RFC 1918), shared CGNAT space (
100.64.0.0/10), loopback, link-local or multicast – plus the historical class for IPv4. - A binary view where network bits and host bits are coloured differently.
- Subnetting: split any network into smaller blocks and list them all, ideal for planning cloud subnets per availability zone.
Accepted input formats
Type 10.0.0.5/24, 10.0.0.5 255.255.255.0, 10.0.0.0/255.255.0.0 or an IPv6 range
such as 2001:db8:abcd::/48. A host address with a prefix is fine: the tool shows the network it belongs to.
Practical examples
Planning an AWS or Azure VPC
Start with 10.0.0.0/16 and split it into /20 blocks: you get 16 subnets of 4,091 usable
addresses each (cloud providers reserve a few more per subnet than the classic two).
Firewall wildcard masks
Network equipment often expects the inverse of the netmask: for /27 the wildcard is
0.0.0.31. The calculator shows both so you can paste the right one.
Your last input is remembered in your browser's localStorage; use Clear Data to forget it.