How to use it
- Type an IPv4 address with a prefix (192.168.1.10/24) or a dotted netmask (192.168.1.10 255.255.255.0), or an IPv6 address with a prefix.
- Read the network, broadcast, host range, counts and masks in the table.
- For IPv4, check the binary view to see where the network part ends.
- To plan subnets, enter how many you need; the tool lists each subnet with its host range.
How it works
For IPv4, the address and mask are handled as 32-bit numbers. The network address is the address with all host bits set to 0, and the broadcast address has all host bits set to 1. Usable hosts are the addresses between them, so a /24 has 256 addresses and 254 usable hosts. Two prefixes are special: a /31 has two usable addresses for point-to-point links (RFC 3021), and a /32 is a single host. The wildcard mask is the inverted netmask, as used in Cisco ACLs and OSPF statements. A dotted netmask is accepted only if its 1 bits are contiguous.
The address type shows whether the address is private (10/8, 172.16/12, 192.168/16), shared carrier-grade NAT space (100.64/10), loopback, link-local, multicast, reserved for documentation, or public. The class (A–E) is shown for reference only; classful addressing has not been used for routing since CIDR was introduced in 1993.
Splitting divides the network into equal subnets. The number you enter is rounded up to the next power of two, because each extra prefix bit doubles the count. Up to 1,024 subnets are listed.
For IPv6, 128-bit arithmetic is done with BigInt so even a /0 is exact. You get the compressed form (RFC 5952: lowercase, leading zeros dropped, the longest run of zero groups replaced with ::), the fully expanded form, the first and last address of the prefix, the number of addresses and, for prefixes up to /64, how many /64 subnets fit. Zone indexes such as %eth0 are ignored.
Examples
Office network
192.168.10.0/23 gives network 192.168.10.0, broadcast 192.168.11.255, hosts 192.168.10.1 to 192.168.11.254 (510 usable), netmask 255.255.254.0 and wildcard 0.0.1.255.
Splitting for VLANs
Enter 10.20.0.0/22 and split into 3. The count is rounded up to 4, giving four /24 networks from 10.20.0.0/24 to 10.20.3.0/24 with 254 hosts each.
IPv6 allocation
A provider assigns 2001:db8:abcd::/48. The tool shows the last address 2001:db8:abcd:ffff:ffff:ffff:ffff:ffff and that 65,536 /64 subnets fit in it.
Frequently asked questions
Why does a /24 have 254 usable hosts and not 256?
The first address identifies the network and the last is the broadcast address, so neither can be assigned to a device. That leaves 256 − 2 = 254.
What is a wildcard mask?
It is the netmask with every bit inverted, for example 0.0.0.255 for 255.255.255.0. Routers such as Cisco use it in access lists and OSPF network statements.
Why can I only split into 2, 4, 8… subnets?
Subnets made by extending the prefix are always equal in size, and each extra bit doubles their number. If you need 5 networks, you get 8 and can leave 3 unused, or plan subnets of different sizes by hand (VLSM).
Does it look up who owns an IP address?
No. Everything is calculated from the numbers you enter. It does not query WHOIS, DNS or geolocation services.
Privacy
All calculations run in your browser. The addresses you enter are not sent anywhere, stored or included in analytics.