Reading the slash
What does CIDR notation mean?
CIDR stands for Classless Inter-Domain Routing. It writes an IP address followed by a slash and a network prefix, such as 192.168.1.0/24. IPv4 addresses have 32 bits: /24 fixes the first 24 as network bits, leaving 8 bits to vary within the block. A larger prefix number means a smaller range.
The address count is 2^(32 - prefix), so a /24 contains 2^8 = 256 addresses. These prefix rules are defined in RFC 4632. Have a dotted-decimal mask instead? Use the subnet mask to CIDR converter to find its prefix.
Two useful directions
How to use the CIDR range calculator
CIDR to IP range
Enter notation such as 192.168.10.77/26. The calculator normalizes it to 192.168.10.64/26 and returns the range through 192.168.10.127.
The prefix leaves 6 variable bits, giving 2^6 = 64 addresses. The entered address, .77, sits inside the aligned .64 through .127 block.
IP range to CIDR
Enter inclusive start and end addresses. If one aligned block cannot represent the range, the calculator returns the smallest exact list of blocks.
Block sizes at a glance
Common CIDR prefixes and IP ranges
Each example shows the complete, inclusive range. Select a CIDR block to load it in the calculator. Address counts are not usable-host counts.
| CIDR block | Start IP | End IP | Total addresses |
|---|---|---|---|
| 172.16.0.0 | 172.16.255.255 | 65,536 | |
| 192.168.1.0 | 192.168.1.255 | 256 | |
| 192.168.1.0 | 192.168.1.63 | 64 | |
| 192.168.1.0 | 192.168.1.3 | 4 | |
| 192.168.1.0 | 192.168.1.1 | 2 | |
| 192.168.1.9 | 192.168.1.9 | 1 |
For equivalent netmasks and wildcard masks, see the subnet mask reference table.
Why alignment matters
Why some ranges need multiple CIDR blocks
A CIDR block always contains a power-of-two number of addresses and must start on a matching binary boundary. For example, a /24 has 256 addresses and starts at an address ending in .0. An arbitrary start or end address usually breaks that alignment, so several smaller blocks are needed to describe the range without including addresses outside it.
This matters when describing an allowlist: rounding 192.168.1.5 through 192.168.1.20 up to 192.168.1.0/27 would also include .0 through .4 and .21 through .31. Use the exact blocks above when those extra addresses must stay outside the range.
Ranges versus hosts
Which addresses can a device use?
This tool counts every address, including the network and final address. For assignable host ranges and broadcast details, use the IPv4 subnet calculator. A /31 can use both addresses on a point-to-point link under RFC 3021; a /32 identifies one address. Provider-specific reservations may further reduce the addresses available to devices.
Working with IPv6? Use the IPv6 subnet calculator; the converter on this page accepts IPv4 only.
Common questions
CIDR and IP range FAQ
What IP range does a CIDR block cover?
It covers every address from the network address through the final address determined by the prefix length. For example, 192.168.1.0/24 includes 192.168.1.0 through 192.168.1.255: 256 addresses in total.
Can I calculate CIDR from an IP address alone?
An IP address alone does not tell you its network prefix. Supply the prefix or subnet mask from your network configuration, or enter the intended start and end addresses in IP range to CIDR mode. To represent only one address, use /32.
Can every IP range be represented by one CIDR block?
No. One CIDR block must contain a power-of-two number of addresses and start on a matching boundary. Other exact ranges require multiple CIDR blocks; this calculator returns the smallest set without missing or extra addresses.
Are total addresses the same as usable hosts?
No. This calculator includes every address in the block. A traditional IPv4 subnet reserves its network and broadcast addresses, so a /24 has 256 total addresses but normally 254 usable hosts. /31 point-to-point links and /32 host routes are special cases, and a hosting provider may reserve additional addresses.