CIDR Aggregation and Route Summarization Explained
Route summarization collapses multiple specific prefixes into one larger advertisement. It reduces routing table size, speeds convergence, and hides network topology from upstream routers.
Route summarization — also called aggregation or supernetting — combines multiple contiguous, specific IP prefixes into a single, less-specific prefix. Instead of advertising eight /24 routes, you advertise one /21. The result is smaller routing tables, faster convergence, and a cleaner topology. Our CIDR aggregation guide covers the full detail; this article provides a practical introduction.
Why Summarize Routes?
The global BGP routing table contains over 950,000 prefixes as of 2025. Without aggregation, every ISP would need to advertise every individual customer subnet separately. Routers have finite memory (the Routing Information Base) and processing capacity. By summarizing, an ISP can represent its entire customer base as a handful of prefixes rather than thousands, significantly reducing the load on internet routers worldwide.
The Binary Math
To summarize multiple networks, find the longest common bit prefix shared by all the addresses. For example, to summarize:
192.168.0.0/24192.168.1.0/24192.168.2.0/24192.168.3.0/24
In binary, the third octet values are 00000000, 00000001, 00000010, 00000011. The first 6 bits (000000) are common across all four. Including the first two octets (16 bits), the total common prefix length is 22 bits. The summary route is 192.168.0.0/22.
Contiguity Requirement
Aggregation only works cleanly with contiguous address blocks that are powers-of-two multiples aligned to the right boundary. You cannot aggregate 10.0.1.0/24 and 10.0.3.0/24 into a single clean prefix — the gap at 10.0.2.0/24 means any summary would also cover addresses you do not control. Misaggregation results in a summary that includes address space owned by someone else, potentially causing routing loops or black holes.
Finding the Summary Route
To find the summary for any set of networks, enter the first and last network addresses into the CIDR calculator and look for the prefix that spans both. Alternatively: convert both addresses to binary, compare bit by bit from the left until they differ, and count the matching bits — that is the prefix length of your summary.
Summary Routes in BGP
In BGP, route summarization is commonly configured at the border router with an aggregate-address statement. The router suppresses the more-specific routes (your individual /24s) and advertises only the summary to upstream peers. Internal routing still uses the specific routes for traffic engineering purposes. This combination of specific internal routing and aggregate external advertisement is standard practice for any organisation with multiple prefixes.