CIDR

Type an IPv4 address and a prefix. Read the network, the mask, the range, and the host count.

CIDR

Network192.168.1.0

Prefix/24

Netmask255.255.255.0

Wildcard0.0.0.255

Broadcast192.168.1.255

First host192.168.1.1

Last host192.168.1.254

Usable hosts254

Addresses256

Learn more about CIDR

What is this tool?

CIDR writes a network as an address and a prefix length. RFC 4632, published in August 2006, describes Classless Inter-Domain Routing. 192.168.1.0/24 means the first 24 bits are the network and the last 8 bits vary.

This page accepts an IPv4 address and a prefix from 0 to 32. It shows the network, the mask, the wildcard, the broadcast, the first and last host, and the counts. A host address such as 192.168.1.50/24 is brought back to its network.

Typical use cases

Checking which network a host address belongs to.
Reading a prefix before writing a firewall rule.
Seeing how many addresses a /28 or a /31 contains.

How to use it

Type an IPv4 CIDR, such as 192.168.1.50/24.
Read the rows. Copy the one you need.
The share link stores the text you typed. The rows are computed again.

The address stays in your browser. The tool does not upload it or look it up. The site keeps your theme, favorites and cookie choice on this device, and it may load a visit counter or ads if you accept them.

Before / after examples

A /24

Before

192.168.1.50/24

After

network 192.168.1.0
netmask 255.255.255.0
first 192.168.1.1
last 192.168.1.254
usable hosts 254

A /31

Before

10.0.0.0/31

After

first 10.0.0.0
last 10.0.0.1
usable hosts 2

Common mistakes

A dotted mask is not a prefix

Invalid

192.168.1.0 255.255.255.0

Valid

192.168.1.0/24

For a prefix of 0 to 30, usable hosts are the addresses minus the network and the broadcast. /31 and /32 do not subtract those two: RFC 3021 uses both addresses of a /31, and a /32 is a single host.

IPv6 is not parsed. An address with a colon shows the error, rather than a wrong IPv4 result.

Good to know

The wildcard is the inverse of the mask. For a /24 it is 0.0.0.255. Some access lists ask for that form instead of the mask.

The share link stores the CIDR text. Anyone who has the link can read that address.

Did you know?

RFC 1519 introduced CIDR in September 1993. RFC 4632, August 2006, made that document historic and is the current description. The old class A, B, and C boundaries are what the prefix length replaced.

RFC 3021, December 2000, says a 31-bit prefix on a point-to-point link may use both addresses. Subtracting a network and a broadcast would leave zero hosts, which is why this page counts 2.

Frequently asked questions

Does the address leave my browser?: The tool does not upload or resolve the address. The site keeps your theme, favorites and cookie choice on this device, and it may load a visit counter or ads if you accept them. A share link puts the CIDR in the URL fragment: that fragment is not sent with the HTTP request, but anyone who has the link can read it.
Why did 192.168.1.50/24 become 192.168.1.0?: The network address clears the host bits. .50 with a /24 sits in 192.168.1.0. The address you typed is still the input.
What is a /0?: Every IPv4 address. The mask is 0.0.0.0, and the usable-host count subtracts the network and the broadcast from 2^32.