CIDR (Classless Inter-Domain Routing) notation writes a network as an address and a prefix length: 172.16.0.0/12, 10.0.0.0/8, 203.0.113.64/26. The prefix says how many leading bits are fixed; every combination of the remaining bits belongs to the block. Cloud firewalls, VPN configurations, routing tables and allow-lists all speak CIDR.
Going from CIDR to a range is mechanical: the first address has all host bits set to 0 and the last has them all set to 1. The other direction is trickier, because only ranges that start on a boundary and contain a power-of-two number of addresses form a single block. The range 10.0.0.1–10.0.0.31, for example, needs five blocks: /32, /31, /30, /29 and /28.
Choose the direction. In CIDR → range mode the calculator also shows the prefix and host bits of your address so you can see where the boundary falls; in range → CIDR mode it lists every block with its first and last address.
How to use the CIDR Calculator
- I want to convert
- IP address
- Prefix (subnet mask width)
- First IP of the range
- Last IP of the range
Everything is calculated in your browser, so results appear instantly and nothing you type is sent to a server.
How it works
In CIDR → range mode the address is rounded down to a multiple of 2^(32 − n). In range → CIDR mode the calculator repeatedly takes the largest block that starts at the current address, is aligned to its size and does not pass the end of the range, then moves on to the next address.
Worked example
172.161.25.102/25: 25 prefix bits leave 7 host bits, so the block holds 2⁷ = 128 addresses. 102 rounded down to a multiple of 128 is 0, so the range is 172.161.25.0–172.161.25.127, mask 255.255.255.128, wildcard 0.0.0.127, with 126 usable hosts. Converting 10.0.0.1–10.0.0.31 back gives five blocks: 10.0.0.1/32, 10.0.0.2/31, 10.0.0.4/30, 10.0.0.8/29 and 10.0.0.16/28 (1 + 2 + 4 + 8 + 16 = 31 addresses).
Prefix sizes
| Prefix | Addresses | Typical use |
|---|---|---|
| /32 | 1 | A single host |
| /30 | 4 | Old point-to-point links |
| /24 | 256 | A small LAN |
| /16 | 65,536 | A campus or VPC |
| /8 | 16,777,216 | Large private network (10.0.0.0/8) |
Formula
Mask = 32 − n host bits · Range = 2³² − n) addresses · First = IP AND mask · Last = first + 2³² − n) − 1 · Range → CIDR: take the largest aligned block that fits, repeat
Worked example
Inputs — I want to convert: CIDR → IPv4 range; IP address: 172.161.25.102; Prefix (subnet mask width): 25.
Results — CIDR = 172.161.25.0/25; First IP (network address) = 172.161.25.0; Last IP (broadcast address) = 172.161.25.127; Subnet mask = 255.255.255.128; Wildcard bits = 0.0.0.127; Addresses in range = 128.
Key terms
- Prefix
- The fixed leading bits of a CIDR block.
- Aligned block
- A block whose first address is a multiple of its size.
- Wildcard bits
- The host part of the mask, written as an inverse mask.
Frequently asked questions
What range does 10.0.0.0/8 cover?
10.0.0.0 to 10.255.255.255 – 16,777,216 addresses.
What does /25 mean?
25 network bits and 7 host bits: 128 addresses, mask 255.255.255.128.
Why does my range need several CIDR blocks?
A single block must start on a multiple of its size and contain a power of two addresses; other ranges must be split.
How many addresses are in a /20?
2¹² = 4,096.
What is 0.0.0.0/0?
Every IPv4 address – the default route.
Is a CIDR block the same as a subnet?
Yes in practice; CIDR is the notation, a subnet is the network it describes.