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Subnet calculator

Enter an IPv4 address in CIDR form (192.168.1.10/24) or with a subnet mask to get its network and broadcast address, host range, host count and masks, in decimal and binary. IPv4 only; runs in your browser.

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Worked example

An office network written as an address with its prefix.

input
172.20.37.9/20
subnet
Address:          172.20.37.9/20
Network:          172.20.32.0
Broadcast:        172.20.47.255
Subnet mask:      255.255.240.0
Wildcard mask:    0.0.15.255
First host:       172.20.32.1
Last host:        172.20.47.254
Total addresses:  4,096
Usable hosts:     4,094
Class (historic): B
Range:            Private (RFC 1918)

A /20 ends inside the third byte, so the network starts at 172.20.32.0, not 172.20.37.0.

Common mistakes

  • Masks that aren’t contiguous

    255.0.255.0 isn’t a valid subnet mask: all the 1 bits must come first. Each mask matches exactly one prefix length.

  • Assuming the network starts at .0

    Only on byte-aligned prefixes like /24. 10.1.5.9/22 belongs to 10.1.4.0, not 10.1.5.0.

  • Giving a host the network or broadcast address

    On /30 and larger subnets, the first and last addresses aren’t usable host addresses.

  • Relying on address classes

    Classes A, B and C were replaced by CIDR in 1993. A 10.x address isn’t automatically a /8; the prefix length decides.

How it works

  1. 1

    Enter an address

    Type 10.0.0.5/20, or an address and a mask like 10.0.0.5 255.255.240.0. A bare address is taken as /32.

  2. 2

    Read the subnet

    You get the network, broadcast, first and last usable host, total and usable addresses, the wildcard mask, and whether the address is private, public or in a special range.

  3. 3

    Split it if you need to

    Choose a longer prefix, or how many subnets you need, to list the smaller networks inside it with their ranges.

Questions

Why are there two fewer usable hosts than addresses?
In a normal subnet the first address names the network and the last is the broadcast address, so neither is given to a host. A /24 has 256 addresses and 254 usable hosts.
What about /31 and /32?
A /31 has just two addresses and no network or broadcast address, so both are usable on a point-to-point link (RFC 3021). A /32 is a single address, used for one host or a route to it.
What is a wildcard mask?
The subnet mask with every bit flipped: 255.255.255.0 becomes 0.0.0.255. Cisco access lists and OSPF use it to say which bits can vary.
Which addresses are private?
10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 (RFC 1918). 100.64.0.0/10 is shared space for carrier-grade NAT, 169.254.0.0/16 is link-local, and 127.0.0.0/8 is loopback; none of these is routed on the public internet.
Does it support IPv6?
Not yet. It handles IPv4 only.