Subnet Calculator

An IPv4 or IPv6 subnet worked out in the browser: its network and host range, broadcast address, masks and reverse DNS zones, the kind of address block IANA says it is, and whether another address is inside it.

Overview

A prefix length splits an address into a network part and a host part. In 192.168.1.10/24 the first 24 bits name the network and the last 8 the host, so every address from 192.168.1.0 to 192.168.1.255 is on the same network and reaches the others directly. The calculator does that split for any address and prefix, IPv4 or IPv6, and names everything that follows from it.

Web interface

Type an address with its prefix or its netmask. The result appears as you type; a complaint about the input waits until you stop typing, because 192.168.1. is not wrong yet. The buttons under the field fill in an example.

• The subnet in CIDR notation stands at the top, with Copy beside it, followed by a sentence placing the address you typed within it.
• A click on any value selects all of it, ready to copy.
• Address type names the IANA block the subnet lies in and links to the IP Address Lookup for the address.
• Is an address in this subnet? takes a second address and says whether it is inside, and if not, which network it belongs to at the same prefix. It also warns when the address is the network or broadcast address, which no host can use.
• In binary, for IPv4, shows the address, netmask and network bit by bit, the network bits in colour.

The address bar follows the input, so a result can be bookmarked or shared as a link.

URL parameters

A netmask in the link is turned into its prefix once the page has read it, so the address bar then shows /calculate/subnet/10.20.30.40/20. An IPv6 address goes into the path as it is, without brackets.

Reading the result

Two IPv4 prefixes have no network or broadcast address to give up. A /31 is a point-to-point link, and RFC 3021 lets both of its addresses be used; a /32 is a single address. IPv6 addresses are shown as RFC 5952 writes them: lower case, no leading zeros, and the longest run of zero groups shortened to ::. The first address of an IPv6 subnet is its Subnet-Router anycast address (RFC 4291).

A reverse zone ends on a whole octet for IPv4 and on a whole hex digit for IPv6, so a prefix in between covers several zones: 172.16.0.0/12 spans the 16 zones from 16.172.in-addr.arpa to 31.172.in-addr.arpa. An IPv4 subnet smaller than a /24 lies inside one /24 zone, and is delegated below it as RFC 2317 describes.

Address types

The address type is the most specific block in the IANA special-purpose address registries that contains the whole subnet, with the first RFC IANA cites for it and two of the registry's columns: Globally reachable, whether packets to the block may travel beyond the network that uses it, and Forwardable, whether a router may forward them at all. A subnet that takes in several blocks, such as 192.0.0.0/16, lists the first four.

Outside every block, an IPv4 subnet is public address space, or multicast within 224.0.0.0/4 (RFC 5771). An IPv6 subnet is global unicast within 2000::/3, multicast within ff00::/8, and otherwise reserved by the IETF (RFC 4291). The tables are the registries as published on 13 September 2026.

IPv4

IPv6

IPv4 prefixes

Input rules

• A prefix follows a slash, 192.168.1.10/24. A netmask follows a slash or a space, 192.168.1.10 255.255.255.0.
• An address on its own is a single host, a /32 or a /128.
• IPv6 may carry an IPv4 address in its last 32 bits, ::ffff:192.0.2.1, and may be written in brackets, [2001:db8::1]/64, the way a URL writes it.
• Upper and lower case are the same.

Some input is refused, each time with the reason:

No API

Privacy

Nothing leaves the browser. The addresses are worked out on the page and are not stored or sent. The address bar changes as you type, which puts the subnet into the browser's history like any other page. The link to the IP Address Lookup opens that tool, which does send the address to the server, and only once you follow it.