Subnet & IP Calculator

Subnet & IP Calculator answers the questions that come up whenever you set up a router, a VPN, a cloud network or a firewall rule. Type an IPv4 address with its prefix, such as 192.168.1.10/24, or with a dotted mask, and it shows the network address, broadcast, first and last usable host, the number of hosts, the subnet and wildcard masks, the address class and what kind of address it is: private, public, loopback, carrier-grade NAT, link-local or a documentation range.

It goes further than most free calculators. The binary view marks network and host bits with weight, underline style and a divider, not just colour. /31 point-to-point links and /32 single addresses are handled properly instead of showing −1 hosts. You can check whether another address falls inside the network, split it by number of subnets, hosts needed or a new prefix (and copy the table as CSV), and get the reverse DNS zone and PTR name for your DNS server.

The IPv6 tab expands and compresses addresses to the standard short form, shows the full range of a prefix, how many /64 networks and /48 sites it holds, the address type and the ip6.arpa zone. All the maths runs in your browser.

1. In the IPv4 tab, type an address into “IP address” with a prefix (10.0.0.5/16) or a mask (10.0.0.5 255.255.0.0), or pick from “Prefix / subnet mask”.
2. Read the Result card: network, hosts and type at the top, then every detail with its own copy button. “Copy all” copies the lot.
3. Use “Is an address in this network?” to test any other address against it.
4. Under “Split into smaller subnets”, choose “Number of subnets”, “Hosts in each” or “New prefix” and read the table; press “Copy as CSV” for a spreadsheet.
5. For IPv6, open the IPv6 tab and type an address with its prefix, such as 2001:db8::/48.

• Accepts CIDR (/24), dotted masks (255.255.255.0) and Cisco-style wildcard masks (0.0.0.255), with a mask picker for /0 to /32.
• Network, broadcast, first and last host, usable and total addresses, mask, wildcard, class, hex and integer forms.
• Correct /31 (RFC 3021) and /32 results.
• Address type: private (RFC 1918), public, loopback, CGNAT (100.64.0.0/10), link-local, multicast, documentation, benchmarking and reserved.
• Binary view of address, mask, network and broadcast with the boundary marked by weight, underline and a divider.
• Reverse DNS zone (including multi-zone and RFC 2317 cases) and the PTR name for the address.
• Subnet splitter by count, hosts per subnet or prefix, showing 256 rows on screen and copying up to 4,096 as CSV.
• IPv6: RFC 5952 short form, full expansion, embedded IPv4, zone IDs, prefix range, /64 and /48 counts, type, multicast scope and ip6.arpa zone.

• Every IPv4 subnet loses two addresses (network and broadcast), so a /24 holds 254 devices, not 256. A /30 holds just 2.
• For a link between two routers, a /31 wastes nothing: both addresses are usable under RFC 3021.
• Home and office networks should use 10.x, 172.16–31.x or 192.168.x. If your router shows a 100.64.x address on its internet side, your provider is using carrier-grade NAT.
• In IPv6, give each LAN a /64 even if it has three devices; automatic addressing (SLAAC) needs it.
• Cloud providers (AWS, Azure, Google Cloud) also reserve a few addresses in each subnet, so usable counts there are slightly lower than shown.