Subnet calculator

IPv4 and IPv6: network, range, host count, masks and reverse-DNS pointers.

Runs entirely in your browser. Nothing you enter here is sent to the server.

This tool needs JavaScript

It does its work in your browser, which is exactly why it cannot fall back to the server: that would mean sending your input somewhere. With scripting off there is nothing for it to run on.

Address with prefix

IPv4 or IPv6, detected automatically. Without a prefix it assumes /32 or /128. A dotted netmask works too: 10.0.0.1/255.255.0.0.

Reading the output

Pure arithmetic, done in this page. Nothing is looked up and nothing is sent anywhere — a subnet calculator that phones home is telling someone about your network layout.

Prefix length: How many leading bits are the network. /24 means the first 24 bits identify the network and the remaining 8 identify hosts.
Network address: The address with every host bit set to zero. Names the subnet.
Broadcast: Every host bit set to one. IPv4 only — IPv6 has no broadcast, it uses multicast instead.
Usable hosts: For IPv4, the range minus the network and broadcast addresses, which is why a /24 holds 254 hosts and not 256. A /31 is the exception: it is two usable addresses, for point-to-point links. A /32 is one single host.
Wildcard mask: The netmask inverted. What Cisco ACLs and OSPF want.
Reverse DNS: The name a PTR lookup for this address uses.

IPv6 is not IPv4 with more digits

Both the compressed and the fully expanded form are shown. Compression follows RFC 5952: lowercase, leading zeros dropped, and :: replacing the single longest run of zero groups — the leftmost one if two runs tie. Tools that compress differently produce strings that look wrong but mean the same address.

Host counts are exact, computed with big integers, so a /64 really does print all twenty digits. A /64 is the standard size for one network segment, and a /56 or /48 is what an ISP hands to a site.

Scope is reported too: global, unique-local (fc00::/7, the rough equivalent of RFC 1918), link-local (fe80::/10, which never routes), loopback, multicast or an IPv4-mapped address.

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