CIDR to IP range
Works in both directions. Give it a CIDR block to see the addresses it covers, or give it a first and last address to get the CIDR blocks that describe exactly that range.
- First address
- 203.0.113.0
- Last address
- 203.0.113.255
- Total addresses
- 256
- CIDR block
- 203.0.113.0/24
When you need each direction
CIDR to range answers “which addresses does this block actually cover?” — the question you have when reading a firewall rule, a cloud provider's published IP list, or a WHOIS record.
Range to CIDR answers the reverse: you have been given a start and end address, and the system you are configuring only accepts CIDR. Allow-lists, security group rules and BGP filters all work this way.
Why ranges rarely map to one block
A CIDR block is a prefix: a fixed set of leading bits, with every combination of the remaining bits. That constrains blocks to power-of-two sizes aligned to their own size. A /24 must start at a multiple of 256, a /20 at a multiple of 4,096.
An arbitrary range chosen by a human almost never lines up with those boundaries, so it splits — often into blocks of wildly different sizes, largest in the middle and small ones at each ragged end.
Common block sizes
| CIDR | Addresses | Typically |
|---|---|---|
| /32 | 1 | A single host, or one allow-list entry |
| /31 | 2 | Point-to-point link (RFC 3021) |
| /30 | 4 | Legacy point-to-point link |
| /29 | 8 | Small business static allocation |
| /24 | 256 | A typical LAN segment |
| /22 | 1,024 | Four /24s, a small ISP allocation |
| /20 | 4,096 | A cloud VPC subnet |
| /16 | 65,536 | A large private network (10.1.0.0/16) |
| /12 | 1,048,576 | The RFC 1918 range 172.16.0.0/12 |
| /8 | 16,777,216 | A legacy Class A, or 10.0.0.0/8 |
Frequently asked
- Why does my range turn into several CIDR blocks?
- CIDR can only describe blocks whose size is a power of two and which start on a multiple of that size. A range like 192.0.2.1–192.0.2.6 satisfies neither, so it decomposes into four blocks: 192.0.2.1/32, 192.0.2.2/31, 192.0.2.4/31 and 192.0.2.6/32. Any firewall or allow-list that takes CIDR needs all four — drop one and you have silently stopped covering part of the range.
- Is the set of blocks the smallest possible?
- Yes. At each step the tool takes the largest block that both starts at the current address and does not overrun the end of the range, which is provably the minimal decomposition. You will not get a shorter list by hand.
- How many addresses does a /0 cover?
- All 4,294,967,296 IPv4 addresses — the entire address space. It is used as a default route rather than as a real allocation.
- Can I paste a range with a dash or dots between the addresses?
- Both work. 192.0.2.1-192.0.2.6, 192.0.2.1 - 192.0.2.6 and 192.0.2.1..192.0.2.6 are all read the same way. If the end address is lower than the start the tool rejects it rather than quietly swapping them, because that is almost always a typo.
Doing this for thousands of addresses?
These calculators run in your browser and never send us your input. When you need geolocation, ASN and network data for addresses in bulk, the API answers in one call — no key required to try it.
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