How to calculate a subnet
- Type an IPv4 or IPv6 address followed by a prefix length or netmask, for example 192.168.1.10/24 or 2001:db8::/32.
- Read the network and broadcast addresses, usable host range, masks, and binary view on the Calculate tab, and copy any value.
- Use Split to divide the network by subnet count, hosts per subnet, or a new prefix.
- Paste addresses or CIDR blocks into IP in range? to see which ones fall inside the network.
The Reserved ranges tab lists the private, loopback, link-local, documentation, and multicast blocks for IPv4 and IPv6, and marks the one your address belongs to. IPv6 is calculated with 128-bit integers, so even a /0 gives exact counts.
Questions
Are the addresses I type sent anywhere?
No. All the maths runs in your browser with plain JavaScript, and the tool makes no network requests. It is safe to use with internal network plans.
Why is a /31 or /32 handled differently?
A normal IPv4 subnet loses two addresses to the network and broadcast addresses. A /31 has two usable addresses (RFC 3021, for point-to-point links) and a /32 is a single host, so the tool counts them that way.
Does IPv6 have a broadcast address?
No. IPv6 uses multicast instead, so every address in an IPv6 subnet is a usable host address. The tool shows the last address of the block where IPv4 shows the broadcast address.
Can I enter a netmask or wildcard mask instead of a prefix?
Yes. Type 10.0.0.1/255.255.255.0 or 10.0.0.1 255.255.255.0. A wildcard such as 10.0.0.1 0.0.0.255 also works. Masks with gaps in the 1 bits are rejected.
How do I split a network into smaller ones?
Open the Split tab and choose a number of subnets, the hosts each subnet needs, or a new prefix length. Subnets are always powers of two, so asking for 5 gives you 8.
Credits
Built by OneTooling using standard browser features. No third-party code runs on your data.