IPv4 Range to CIDR Expander Online — Free
Runs 100% in your browser — your files never leave your device.
Cloud consoles and firewall rules often ask for CIDR when all you have is a start and end address. Enter both ends of your range and get the enclosing CIDR notation, the expanded start and end, and how many addresses you gain by rounding up to a block boundary.
How it works
- 1
Enter start and end
Two plain IPv4 addresses define your range.
- 2
Read the enclosing CIDR
The tool rounds your range up to the nearest block boundary.
- 3
Compare sizes
See the original address count next to the expanded one.
About this tool
What range expansion does
Give the tool a start and end IP, and it computes the full list of addresses between them — or, more usefully, the tightest set of CIDR subnets that exactly covers that range without including anything outside it.
The use case: allowlists and routes
Firewall rules and route tables speak CIDR, not arbitrary ranges. An allowlist entry for a vendor whose addresses span 203.0.113.7 to 203.0.113.14 must expand into the correct subnets — too wide and you allow strangers, too narrow and you miss addresses. The expander computes the exact minimal cover.
Why the minimal cover matters
Any range can be covered by including a bigger block — the art is including nothing extra. The expander outputs only subnets fully contained in your range, so the resulting rules are as narrow as the range allows.
Frequently asked questions
How large a range can it expand?
Thousands of addresses process instantly; the CIDR output stays compact regardless — subnets summarize ranges by nature.
Why does the output include subnets that look wider than my range?
The subnets align to power-of-two boundaries — a range crossing one will produce overlapping subnets that together cover exactly the range, no more.
Can I expand a single-address range?
Yes — a single address is the /32 subnet of itself, the tightest possible cover.