IPTools UK

CIDR Calculator

Convert CIDR notation into an address range, get the total address count, and check whether a specific IP falls inside the block.

About this tool

CIDR notation is the standard way to describe an IP address range compactly. Rather than listing a full subnet mask, it states the address followed by a prefix length — the number of fixed leading bits. This tool converts that notation into concrete start and end addresses, works out how many addresses the block contains, and can check whether a specific address belongs to it.

How to use this tool

  1. Enter a CIDR block, e.g. 10.0.0.0/16 or 2001:db8::/32.
  2. Optionally, enter an IP address to check whether it falls inside that block.
  3. Select "Calculate". The URL updates with your input, so results can be bookmarked or shared.

Frequently asked questions

What does CIDR notation mean?
CIDR (Classless Inter-Domain Routing) notation combines an IP address with a slash and a prefix length, e.g. 192.168.1.0/24. The prefix states how many leading bits are fixed for the network, leaving the rest for host addresses.
What is the difference between this and the subnet calculator?
This tool focuses on converting CIDR notation to an address range and count (for both IPv4 and IPv6), and checking whether a given address falls inside a range. The subnet calculator additionally shows IPv4-specific detail such as the subnet mask, wildcard mask and binary breakdown.
Does this work for IPv6 CIDR blocks?
Yes. Enter an IPv6 address and prefix, e.g. 2001:db8::/32, and the tool will calculate the first and last address and the total address count (shown as an exact figure, since IPv6 ranges can vastly exceed normal number limits).

Subnet Calculator

Calculate network address, broadcast address, usable host range, subnet mask, wildcard mask and binary representation for any IPv4 subnet.

IP Converter

Convert IPv4 addresses between dotted-decimal, hexadecimal and binary, and expand or compress IPv6 addresses.

IP Lookup

Look up an IPv4 or IPv6 address: version, approximate location, ASN, network organisation, reverse hostname and RDAP registration data.