IP Subnet Calculator (IPv4 and IPv6)

Give it an address with a prefix — 192.168.1.10/24, 10.20.30.40 255.255.224.0 or 2001:db8:abcd:12::7/64 — and it works out the block that address belongs to: the network address, the last (broadcast) address, the netmask and wildcard, the range of usable hosts and exactly how many addresses the block holds. IPv6 counts are exact, not rounded: a /64 holds 18,446,744,073,709,551,616 addresses and the page says so. It also names what kind of address it is — private, shared (CGNAT), documentation, link-local, unique local and so on — from the IANA special-purpose registries, gives the reverse-DNS zone when the block falls on a boundary, splits the block into smaller subnets, and tells you whether another address is inside it.

1. Type an address and a prefix length (/24), or an IPv4 address and a netmask (255.255.255.0). IPv6 is written as usual, with :: for a run of zeros.
2. Read the network, broadcast or last address, netmask, wildcard and usable range on the right.
3. Type another address under “Is this address inside the block?” to check it against the range.
4. Choose a smaller prefix under “Split into smaller blocks” to list the subnets the block divides into.

• It calculates what a block is. It does not tell you which addresses are in use, who owns a public address, or how a particular router is configured.
• Address types follow the IANA IPv4 and IPv6 special-purpose registries as of September 2026. An address shown as public or global unicast may still be unallocated.
• Leading zeros in IPv4 (such as 010.1.1.1) are refused on purpose: some software reads them as octal and some as decimal.
• Splitting lists at most the first 256 blocks and offers prefixes up to 16 bits longer than the block.
• The arithmetic runs in your browser. Nothing you type is sent anywhere, and no address is looked up on the internet.

254. A /24 has 256 addresses; the first is the network address and the last is the broadcast address, and neither can be given to a host. In general an IPv4 block of prefix p has 2^(32−p) addresses and 2^(32−p) − 2 usable hosts.

RFC 3021 lets a /31 be used for a point-to-point link between two routers. With only two addresses there is no room for a separate network and broadcast address, so both are hosts. A /32 is a single host, often used for a loopback or a route to one machine.

The netmask with every bit flipped: 255.255.255.0 becomes 0.0.0.255. Cisco access lists and OSPF use it to say which bits of an address may vary. It is not the same as a netmask, and mixing them up is a common configuration mistake.

IPv6 replaced broadcast with multicast. Every address in an IPv6 block can be used, so the last address is simply the last address. Subnets for ordinary networks are almost always /64, because stateless address autoconfiguration (SLAAC) needs 64 bits for the interface.

Shared address space from RFC 6598, used by internet providers for carrier-grade NAT between their equipment and your router. If your router’s WAN address starts with 100.64 to 100.127, you are behind your provider’s NAT, which is why port forwarding may not work.

For IPv4, the three RFC 1918 ranges: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. For IPv6, unique local addresses in fd00::/8 — choose a random 40-bit global ID so two networks joined later do not clash. None of these are routed on the public internet.