Subnet Summary Calculator

Calculator

Mode Union then summarize

Computes union coverage, then emits minimal CIDRs.

Include merged spans Yes No

Shows intermediate union spans for verification.

Quick examples

Load example A Load example B

Subnets, ranges, or IPs (one per line)

Accepted formats: a.b.c.d/prefix, a.b.c.d, startIP-endIP. IPv4 only.

Summarize Reset

Example Data Table

Formula Used

• CIDR to range: network = ip AND mask, broadcast = network + (2^(32-prefix) - 1).
• Union merge: sort ranges by start, then merge overlaps and adjacencies.
• Range to minimal CIDRs: choose the largest aligned power-of-two block at the current start that fits the remaining range.
• Prefix from block: prefix = 32 - log2(blockSize).

How to Use This Calculator

1. Paste your IPv4 subnets, ranges, or IPs, one per line.
2. Click Summarize to compute the union coverage.
3. Review the summarized CIDR list and optional merged spans.
4. Export to CSV or PDF for documentation or route planning.

FAQs

1) What does subnet summarization mean?

It combines many networks into fewer, larger CIDR routes. The goal is fewer entries while still covering the same IP addresses.

2) Does this tool handle overlaps?

Yes. Overlapping CIDRs or ranges are merged into a single union span before summarization, so duplicates never inflate totals.

3) What about adjacent networks?

Adjacencies are merged too. If one range ends exactly where the next begins, the tool treats them as continuous coverage.

4) Will the output always be one supernet?

Not always. If your inputs contain gaps, the union becomes multiple spans. Each span is summarized separately into minimal CIDRs.

5) Why can’t I summarize across gaps?

Summarizing across gaps would include addresses you did not specify. That can create unintended routing, filtering, or audit issues.

6) How are netmask and wildcard calculated?

Netmask sets the first prefix bits to one. Wildcard is the host mask, equal to 2^(32-prefix) minus one, shown in dotted form.

7) Is IPv6 supported?

This version focuses on IPv4 because IPv6 summarization needs 128-bit arithmetic. You can still summarize IPv4-only datasets here.

8) What’s the best way to verify results?

Compare the merged spans with expected coverage, then validate the CIDR list on a router, firewall, or an offline subnet worksheet.