Supernetting Calculator

Imagine your router has a long list of 50 separate delivery addresses it needs to remember. Supernetting is like telling the router — instead of remembering all 50 addresses individually, just remember one postcode that covers all of them. Multiple smaller networks are combined into one single summary route, making your router's job faster and simpler.

Network engineers use this technique to shrink routing tables, reduce the memory routers need, and make networks run more efficiently. It is also called Route Summarization or IP Aggregation.

Not every group of networks can be summarized. Before you enter your networks, check these three rules.

1. 1 Networks must be contiguous — no gaps allowed.
   Your networks must sit right next to each other in the address space with no missing blocks in between.
   ✔ Good: 192.168.0.0/24, 192.168.1.0/24, 192.168.2.0/24, 192.168.3.0/24
   ✘ Bad: 192.168.0.0/24, 192.168.2.0/24 — there is a gap at 192.168.1.0
2. 2 The number of networks should be a power of 2.
   Summarize 2, 4, 8, or 16 networks at a time — not 3, 5, or 7.
   ✔ Good: 2 networks, 4 networks, 8 networks
   ✘ Avoid: 3 networks, 5 networks, 6 networks
3. 3 The first network must start on the right boundary.
   The starting address must be evenly divisible by the total block size you are summarizing.
   ✔ Good: 4 × /24 starting at 192.168.0.0 → clean /22 summary
   ✘ Bad: 4 × /24 starting at 192.168.1.0 → the summary spills outside your intended range

Type or paste the networks you want to summarize into the text box. Each network goes on its own line in CIDR format.

1. 1Click inside the Networks to Summarize text box.
2. 2Type each network on a new line in CIDR format, for example:
   192.168.0.0/24
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
3. 3You must enter at least two networks. The calculator sorts them automatically before processing.

Click Calculate Summary Route. Results appear in two tables — the summary route and a breakdown of each individual network included.

1. →Summary Network — The single combined route to advertise to your router.
2. →First Usable IP / Last Usable IP — The assignable address range inside the summary block.
3. →Broadcast — The last address of the summary block.
4. →Total Usable Host — Usable addresses in the summary block (block size − 2).
5. →Total Available Host — Full block size including network + broadcast addresses.
6. →Netmask / Wildcard — Ready to enter directly into your router configuration.
7. →Individual Networks table — Confirms exactly which blocks are included inside the summary.

Example — summarizing 4 × /24 networks into one summary route

Over-summarization happens when your summary route accidentally covers more addresses than you intended. This occurs when networks do not start on the correct boundary for the block size being summarized.

Example of over-summarization — networks that cross a boundary

Supernetting is the process of combining multiple smaller networks into one single larger summary route. The main reason engineers use it is to shrink routing tables — a smaller routing table means routers use less memory, make faster forwarding decisions, and generate less traffic when sharing routing information. It is especially common in large ISP and enterprise networks.

Total Usable Host is the number of addresses assignable to devices in the summary block — calculated as 2^{(32−prefix)} − 2, subtracting the network and broadcast address. Total Available Host is the full block size — 2^{(32−prefix)} — counting all addresses including those two reserved ones. For a /22 summary: Total Usable Host = 1,022, Total Available Host = 1,024.

They are opposite operations. Subnetting starts with one large network and breaks it into smaller pieces — for example splitting a /24 into four /26 subnets. Supernetting does the reverse — it takes multiple smaller networks and combines them into one larger summary — for example merging four /24 networks into a single /22. Subnetting divides; supernetting combines.

Contiguous means the networks must sit right next to each other in the address space with no gaps. If there is a gap, the summary route would have to stretch further than intended to cover both sides — pulling in addresses that do not belong to you. The calculator automatically checks for this and will show an error if your networks are not contiguous.

All IP address blocks work in powers of 2 — every step up in prefix length doubles the block size. Clean summaries can only be made from groups of 2, 4, 8, 16, or 32 networks. If you try to summarize 3 networks, there is no single prefix that covers exactly 3 — the nearest option covers 4, so the summary will always include one extra block you did not intend.

Over-summarization happens when your summary route accidentally covers more addresses than you own or intend to advertise. It occurs when your group does not start at the correct boundary. To avoid it, make sure the first network in your group starts at an address evenly divisible by the total block size you are summarizing.

Enter each network in standard CIDR format — the network address followed by a forward slash and the prefix length, for example 192.168.0.0/24. Put each network on its own line. You must enter at least two networks. You do not need to sort them — the calculator arranges them automatically.

Yes, the calculator accepts networks with different prefix lengths. However, getting a clean and accurate summary becomes much harder when block sizes differ because boundaries rarely align perfectly. In practice, most real-world summarization uses networks of the same size — all /24s or all /25s — because they naturally align at the correct boundaries.

After the summary route is calculated, the Individual Networks table shows a breakdown of every network you entered — its own network address, usable host range, broadcast, netmask, and wildcard. This is useful for double-checking that all input networks are correctly included inside the summary, and for keeping a record of the individual routes the summary is replacing.