CIDR Calculator
Paste a list of CIDR blocks, IPv4 or IPv6, to see each block’s address range, size and type, and to get the shortest list that covers exactly the same addresses. Overlapping and adjacent blocks are merged, which is what routers call route summarisation and what firewall and allow-list maintainers need to keep rules short.
- Runs in your browser
- No sign-up
- Free to use
One block per line, IPv4 or IPv6. Each is analysed, and overlapping or adjacent blocks are combined into the smallest equivalent list.
| Block | Range | Addresses | Type |
|---|
How to use CIDR Calculator
- Paste CIDR blocks, one per line.
- Read the range, size and type of each block.
- Copy the aggregated list.
- Use it in route tables, firewall rules or allow-lists.
CIDR Calculator features
IPv4 and IPv6
Both families in one list, summarised separately.
Range and size
First and last address and number of addresses.
Type
Private, public, documentation, CGNAT, link-local and other special ranges.
Aggregation
Overlapping and adjacent blocks merged into the fewest CIDRs.
Host-bit warning
Flags blocks written with host bits set, such as 10.0.0.5/24.
Private
Calculated in your browser.
When to use CIDR Calculator
- Shortening firewall or security-group rules.
- Summarising routes before advertising them.
- Merging cloud provider IP lists.
- Checking whether address blocks overlap.
CIDR Calculator FAQ
What is CIDR notation?
An address followed by a slash and the number of fixed leading bits, such as 10.0.0.0/24, which covers 256 addresses from 10.0.0.0 to 10.0.0.255.
What does aggregation do?
It replaces blocks that touch or overlap with larger blocks covering exactly the same addresses: 10.0.0.0/25 and 10.0.0.128/25 become 10.0.0.0/24.
Will aggregation ever add addresses?
No. The result covers exactly the same addresses as the input, never more.
What does “host bits set” mean?
The address has bits beyond the prefix, as in 10.0.0.5/24. Most systems treat it as 10.0.0.0/24; some reject it.
Can I mix IPv4 and IPv6?
Yes. Each family is summarised on its own.
Is anything sent?
No.
Fewer, larger blocks
CIDR, classless inter-domain routing, describes address blocks by a prefix length rather than the old fixed classes. Any block whose size is a power of two and which starts on a matching boundary can be written as one CIDR, from a single address (/32) to the whole internet (/0).
Lists of blocks tend to grow piecemeal: a range added here, another there, some overlapping, some adjacent. Router tables, firewall rule sets and cloud security groups all work better with fewer entries, and some have hard limits. Aggregation finds the minimal set of CIDRs that covers exactly the same addresses.
The algorithm merges overlapping and adjacent ranges and then expresses each merged range as the fewest aligned blocks. Adjacent blocks only combine into one larger CIDR when they are aligned: 10.0.1.0/24 and 10.0.2.0/24 are neighbours but cannot become a single /23, because a /23 must start on an even third octet.
Knowing the type of each block matters too. Private ranges should never appear in public routing; documentation ranges belong only in examples; carrier-grade NAT space is not reachable from the internet. The type column flags these at a glance.