CIDR / Subnet Calculator

Enter a CIDR block to calculate subnet details including network address, broadcast, usable range and subnet mask. All processing happens in your browser.

About CIDR / Subnet Calculator

CIDR (Classless Inter-Domain Routing) notation combines an IP address with a prefix length to define a network. This tool calculates all subnet details from a CIDR block. All processing runs in your browser.

  • Network Address: The first address in the subnet, identifies the network itself.
  • Broadcast Address: The last address in the subnet, used for broadcasting to all hosts.
  • Usable Range: Addresses available for host assignment (excludes network and broadcast).
  • Subnet Mask: Determines which bits identify the network vs. host portion.

The number after the slash is how many bits are fixed, which is why a smaller number means a larger network. Two of the addresses in any ordinary subnet are not usable: the first identifies the network and the last is the broadcast address, so a /24 gives you 254 hosts rather than 256. Point to point links are the exception, where a /31 is used deliberately and both addresses are usable.

Related tools

IP Address Lookup DNS Lookup Number Base Converter

Something wrong out at the edge?

A lookup answering from the wrong record, a certificate that expires on a Sunday, a header that gives away more than it should. We look after servers, DNS and the delivery layer, and we set them up so the next surprise is caught before your users find it.

Ask about your infrastructure

Frequently Asked Questions

Why does a /24 have 254 usable addresses and not 256?
The first address names the network itself and the last is the broadcast address, so neither can be assigned to a host. That subtraction applies to every ordinary subnet, which is why a /30 gives you two usable addresses out of four.
Which way round is a bigger prefix?
A bigger number is a smaller network. The prefix counts the bits that are fixed, so /24 fixes twenty-four and leaves eight for hosts, while /25 fixes one more bit and halves the space.
What is a /31 for?
A link between exactly two routers. There is nothing to broadcast to, so the standard allows both addresses to be used, which saves two addresses on every point to point link in a network.
How do I split a network into smaller ones?
Add bits to the prefix. Each bit you add doubles the number of subnets and halves their size, so a /24 becomes two /25s, four /26s and so on. Plan the split before you assign anything, because renumbering later is painful.
Are the private ranges fixed?
Yes: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 are reserved for private use and are never routed on the public internet. Using anything else internally means you may one day collide with a real destination.