Guide
Summarising routes without advertising addresses you do not own
A router that carries a separate route for every branch network has a larger table, more updates and more chances for instability. Route summarisation replaces many routes with one that covers them all. It is also the place where mistakes creep in: a summary that is too short quietly claims addresses that belong to someone else. This page shows both the single summary and the exact list, side by side, with the cost in extra addresses.
How the common prefix is found
Write the networks in binary and compare them from the left. The summary prefix ends where the first difference appears among any of them. For 192.168.0.0/24, 192.168.1.0/24, 192.168.2.0/24 and 192.168.3.0/24, the first 22 bits are identical and the 23rd varies, so the summary is 192.168.0.0/22. Four consecutive /24 networks starting on a multiple of four fill a /22 completely, so no extra addresses are covered, which is the ideal case.
Why a gap matters
If one network is missing, the shortest single prefix still has to span the gap. Take 10.1.0.0/24, 10.1.1.0/24 and 10.1.3.0/24: the lowest and highest differ already in the 23rd bit, so the covering prefix is 10.1.0.0/22, which is 1,024 addresses while the three networks hold 768. The 256 addresses of 10.1.2.0/24 are claimed without being yours. If that network belongs to another department or customer, traffic for it is pulled to the wrong router. The page shows this as “Extra addresses covered” with a warning.
The exact list
When a single summary would cover extra space, the other result is what you want. The exact list merges neighbours that line up and overlapping blocks and keeps the rest separate. For the gap example it is 10.1.0.0/23 plus 10.1.3.0/24, which advertises exactly the three original networks in two routes instead of three. Where the page reports 0 extra addresses, the single summary and the exact list are the same.
Reading the output
The panel lists the summary route, the exact minimal list, the addresses in your networks, the addresses in the summary and the extra addresses. A short note says whether the summary can replace your networks without side effects. Networks can be IPv4 or IPv6, one per line, but not both in the same list, and a plain address counts as a single-host network. Overlapping and nested entries are merged in the exact list.
A worked example
The default sample is the four /24 networks from 192.168.0.0 to 192.168.3.0. The single summary is 192.168.0.0/22 and the exact list is the same block; both hold 1,024 addresses, so there are 0 extra addresses, and the page confirms the summary can replace the networks without side effects. Load the gap sample and the single summary grows to 1,024 addresses against 768 entered, with 256 extra.
Limits
The page performs the arithmetic. It cannot know your topology, whether the gap addresses exist elsewhere or whether the summary will black-hole traffic. On real equipment, check how your protocol handles summaries (for example discard routes) and test before advertising. To turn a start and end address into blocks, see IP Range to CIDR.
Related tools
- VLSM Calculator plans the subnets you will later summarise.
- IP Range to CIDR covers an arbitrary range exactly.
- CIDR to IP Range shows the first and last address of any block.
- Subnet Calculator explains each network in the list.
Checked against the current version of the Route Summarization Calculator on .