How to use the CIDR Calculator
- Type a CIDR block, such as
172.16.0.0/20, in the first box. - Read the first and last address, the number of addresses and the masks in the results.
- To go the other way, scroll to IP range to CIDR and enter the first and last IP of your range.
- Copy the list of CIDR blocks for your firewall, AWS or Azure rules.
What is CIDR notation?
CIDR (Classless Inter-Domain Routing) writes a block of addresses as a starting address and a prefix length: 10.0.0.0/22 means "the 1,024 addresses that share the first 22 bits with 10.0.0.0", from 10.0.0.0 to 10.0.3.255. It replaced the old class A, B and C networks in 1993, so networks can be any power-of-two size.
Why a range can need several blocks
A CIDR block always holds a power of two (1, 2, 4, 8, 16…) addresses and must start on a matching boundary. A range like 10.0.0.5 to 10.0.0.20 doesn't fit one block, so it becomes several: 10.0.0.5/32, 10.0.0.6/31, 10.0.0.8/29, 10.0.0.16/30 and 10.0.0.20/32. The calculator always finds the shortest list.
Frequently asked questions
How many IP addresses are in a /22?
1,024 (2 to the power of 32 − 22). Usable hosts: 1,022.
What does /32 mean?
A single IP address. In IPv6, a single address is /128.
What does 0.0.0.0/0 mean?
Every IPv4 address. In firewall rules it means "anywhere"; the IPv6 equivalent is ::/0.
Updated