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FREE TOOL
Paste a list of IPv4 prefixes, one per line. The tool returns two different answers. The exact summary is the fewest prefixes that cover exactly the addresses in your list. The covering supernet is the one prefix that holds all of them, and when it also holds addresses that are not in your list, the tool lists those extra ranges as prefixes. The summary runs in this page, and the page does not send what you enter to a server. No account is required.
This tool runs in your browser and needs JavaScript turned on. The boxes stay locked until the page script loads, so the page cannot send anything you enter. If they stay locked, the script did not load. Turn on JavaScript or reload the page to use the tool.
The tool is ready. It runs in this page, and the page does not send what you enter to a server.
These prefixes cover every address in your list and no other address.
| Prefix | First address | Last address | Addresses |
|---|
None. The covering supernet holds exactly the addresses in your list, so it is the same prefix as the exact summary.
The covering supernet also holds these addresses, and none of your prefixes include them. A route for the supernet claims them too.
| Prefix | First address | Last address | Addresses |
|---|
Each step shows the 32 bits in binary and dotted decimal. Network bits are shown in bold and host bits in normal weight where a prefix applies.
A prefix covers a block of addresses whose size is a power of two and which starts on a multiple of that size. A list of prefixes can be replaced by fewer, shorter prefixes in two different ways, and the difference decides whether a router advertises addresses you do not have.
The exact summary keeps the same addresses. The tool clears any host bits, drops a prefix that repeats another or sits inside another, joins prefixes that overlap or sit next to each other into continuous runs, and splits each run into the fewest aligned prefixes. The result covers every address in your list and nothing else. When the list happens to fill one aligned block, such as 172.16.0.0/24 through 172.16.3.0/24, the exact summary is the single prefix 172.16.0.0/22.
The covering supernet is the longest single prefix that holds every prefix in the list. The tool finds it from the lowest and highest addresses: the bits they share, counted from the left, are the supernet's network bits. It is always one prefix, but it can hold addresses that none of your prefixes include. For 10.1.0.0/24 and 10.1.2.0/24, the covering supernet is 10.1.0.0/22, which also holds 10.1.1.0/24 and 10.1.3.0/24. The tool lists that extra coverage as prefixes every time, so you can see what the shorter route would also claim.
To practice, work out the exact summary and the covering supernet for 10.1.0.0/24 and 10.1.2.0/24 on paper first, in binary. Then enter the two prefixes here and compare your bits with the working.
A router that advertises a summary tells its neighbors it can reach every address in that prefix, so traffic for the extra ranges is sent to it as well. RFC 4632, section 5.1, says that the router which generates an aggregate route must discard packets that match the aggregate but none of its more specific routes, and that the aggregate's next hop should be the null destination. The RFC gives this rule to prevent forwarding loops when some of the covered addresses are not reachable. The traffic for the extra addresses is then dropped at the router that summarized rather than looping, but it is still dropped. Use the covering supernet only when the extra addresses are unused or are also yours to route.
A prefix that repeats another line, or that sits inside a shorter prefix on another line, adds no addresses. The tool keeps it in the count of prefixes entered, says which line already covers it, and leaves it out of the summary. A prefix written with host bits set, such as 10.1.1.5/24, is read as its network, 10.1.1.0/24, and the result says so.
The tool works on IPv4 prefixes only. It summarizes the list you give it and does not know which routes a router actually has, which neighbor learned them or which of them are reachable. A routing protocol also has its own rules about where a summary can be made. In OSPF, for example, area border routers summarize an area into the backbone by advertising one route for each configured area address range, and the tool does not model those rules. Treat the result as the arithmetic, and check the extra coverage against your address plan before you configure a summary.
Sources: RFC 4632, section 3.1, for prefixes as power-of-two blocks; RFC 4632, section 5.1, for discarding packets that match an aggregate but no more specific route; RFC 2328, section 3.5, for OSPF area address ranges summarized at area border routers.