Subnet and PortsCIDR to IP range converter
A CIDR block is a start address and a prefix length. This tool shows the range that it covers.
How a CIDR block maps to a range
In CIDR notation (RFC 4632), 203.0.113.0/24 means: the first 24 bits are the network and the remaining 8 bits are free. With 8 free bits there are 2^8 = 256 addresses, from 203.0.113.0 (all free bits zero) to 203.0.113.255 (all free bits one). A bigger number after the slash means a smaller block: /16 has 65,536 addresses, /8 has 16,777,216 and /32 has one.
Using the result
- Firewalls and allow lists. Many tools accept CIDR blocks. Use the shortest list of blocks that covers exactly what you want.
- Checking a log. Convert a block from a notice or a blocklist to a range and see whether an address you saw lies inside.
- Planning. Check how many addresses a block provides before you ask for it.
If you want to go the other way, from a range of addresses to blocks, use the range to CIDR tool.
If the address you type is inside the block
The tool shows the block that your address belongs to. For 203.0.113.77/24 the block starts at 203.0.113.0. Many systems reject a block whose address has bits set after the prefix, such as 203.0.113.77/24, while others quietly fix it. Write blocks with the network address.
The size of common blocks
| Prefix | Addresses | Example use |
|---|---|---|
| /8 | 16,777,216 | An old large allocation, or the private block 10.0.0.0/8 |
| /16 | 65,536 | A large organisation, or the private block 192.168.0.0/16 |
| /24 | 256 | A typical LAN |
| /30 | 4 | A link between two routers (2 usable hosts) |
| /32 | 1 | One host |
Common mistakes
- Thinking /24 means 24 addresses. It means 24 fixed bits.
- Mixing the order: in
192.168.0.0/16the prefix 16 is the length of the network part, not the number of hosts. - Listing a range as a block when it is not aligned: 192.168.1.10 to 192.168.1.200 is not one block (use the range tool).
How many addresses does a /27 have?
2^(32-27) = 2^5 = 32 addresses, 30 of them usable hosts in an ordinary subnet.
Does the tool work for IPv6?
Yes. Type a block such as 2001:db8::/32.
This is an independent guide. It is not connected with IANA, the IETF or any network equipment maker. Address and port data come from the IANA registries as fetched on 14 November 2026; RFC texts were read from rfc-editor.org. Check the originals for anything you rely on.