CIDR Range Calculator

How to use

1. 1Pick a mode at the top: Expand CIDR (CIDR to range), Range to CIDR (the reverse), or Contains test (is an IP inside a CIDR).
2. 2In Expand, paste one CIDR per line, or comma or semicolon separated. Mix IPv4 and IPv6 freely; switch the IPv6 format chip between compressed and fully expanded for the format your tooling expects.
3. 3In Range to CIDR, paste IPv4 entries (10.0.0.5, 10.0.0.0/24, or 10.0.0.5-10.0.0.250) one per line. The tool merges overlapping ranges and emits the smallest set of CIDR blocks that cover the same addresses.
4. 4In Contains, enter a CIDR block on top and a list of IPs to test below. Each IP is reported inside or outside, with the offset from the network start for inside hits.
5. 5Use Copy table, Copy CIDR list, or Copy report to grab a tab-separated or newline-separated result block for spreadsheets, change tickets, or pull requests.

About this tool

CIDR Range Calculator is a browser-based networking tool that handles the three jobs people search for when they paste a CIDR into Google: expand a CIDR block (or a paste of many CIDR blocks) to the first address, last address, total address count, and netmask; reverse the direction by turning a free-form list of IPv4 addresses, ranges, and CIDRs into the minimum set of CIDR blocks that exactly cover the same addresses; and contains-test a list of IPs against a CIDR block to find which addresses fall inside an allow-list, ACL, firewall rule, or routing entry. Expand mode supports both IPv4 (parsed as unsigned 32-bit integers) and IPv6 (parsed and computed with BigInt so the 128-bit space is exact, with RFC 5952 compressed output by default and a fully expanded option for tools that need eight 16-bit groups). It also tolerates a paste of mixed CIDRs separated by newlines, commas, or semicolons, flags any line with host bits set so you can see when 10.0.0.5/24 was meant to be 10.0.0.0/24, and reports the total address count exactly as a thousand-separated number for small blocks and as an approximate power of ten for huge IPv6 prefixes. Range to CIDR mode accepts plain IPv4 addresses, dashed ranges like 10.0.0.5-10.0.0.250, and CIDRs in the same paste, merges overlapping or adjacent ranges into the smallest set of non-overlapping intervals, then runs the standard greedy algorithm (largest aligned block that fits, then recurse on the remainder) to emit the optimal CIDR cover. This is the canonical answer for compressing a long list of allow-listed addresses into the smallest firewall configuration, normalizing scraped ranges before importing them into a routing table, or sanity-checking that two ranges overlap. Contains mode validates each IP, checks it against the CIDR, reports inside or outside, and shows the offset from the network start for inside hits so you can identify exactly where a specific host falls inside a /16 or a /48. Useful when you are debugging a firewall rule, checking whether a logged client IP is in your office VPN range, sizing a VLAN against a planned address pool, normalizing CIDRs pasted from another tool, or compressing an export of allow-listed IPs into the fewest firewall lines possible. All parsing, bit math, BigInt arithmetic, and rendering happens locally in your browser, so the production CIDR plans, IP allow-lists, and ACL drafts you paste here never leave your device.

Free to use. Works in your browser. No signup, no login.