Subnet Merge Calculator

Calculator

Subnet merging is based on address ranges. A CIDR block A/B represents a network range:

• Netmask: mask = 1…1 0…0 with B leading ones.
• Network address: network = ip AND mask
• Block size: size = 2^(32 − B)
• Broadcast: broadcast = network + size − 1

Two equal-size blocks can supernet when they are adjacent, aligned, and share the same first B−1 bits. The merged prefix becomes B−1.

1. Paste subnets, one per line, using CIDR or IP plus netmask.
2. Choose output options for exact aggregation or a single summary.
3. Click Merge Subnets to display results above the form.
4. Download CSV or PDF for reports, audits, or documentation.

1) What does “exact aggregation” mean?

It produces CIDR blocks that cover only the input address space, without adding extra addresses. This is safest for audits, firewall rules, and inventory lists.

2) Why can a “summary supernet” include extra addresses?

A single supernet must be a power-of-two block aligned on its boundary. If your inputs don’t fill that block perfectly, the smallest covering supernet also covers unused addresses.

3) Can I paste host IPs like 192.168.1.10/24?

Yes. The tool normalizes them by applying the netmask and converting to the correct network address, so the calculation remains consistent.

4) How do adjacent subnets merge?

They merge when their ranges touch and the combined range is aligned to a larger CIDR boundary. Two adjacent /24 blocks can merge into one /23 only when the /23 starts on the correct even boundary.

5) What happens if my inputs overlap?

Overlaps are detected and shown as a warning. Exact aggregation still works because overlapping ranges collapse into the union, but overlapping inputs may indicate duplicate entries or inconsistent planning.

6) Does the calculator support IPv6?

This version focuses on IPv4 for precise range math and common routing summaries. You can extend it for IPv6, but range-to-CIDR logic and parsing would need 128-bit handling.

7) Why are usable hosts zero for /31 and /32?

Usable host counts depend on whether network and broadcast addresses exist. /32 is a single address. /31 is commonly used for point-to-point links and does not reserve network/broadcast in many modern networks.

8) How are the exported files generated?

CSV contains the merged CIDRs and key fields. PDF is a compact report listing the exact merged blocks and optional summary supernet, suitable for sharing or archiving.