Supernet Calculator

What Is Supernetting?

Supernetting, also known as route summarization or prefix aggregation, is the process of combining multiple contiguous subnets into a single, larger CIDR block. While subnetting divides a network into smaller pieces, supernetting does the opposite: it merges smaller networks into a larger one. This is a critical technique for managing routing table growth on the internet and within large enterprise networks.

In practice, supernetting directly reduces the memory and CPU overhead on routing devices. A router that receives a single aggregate route instead of dozens of more specific prefixes performs fewer longest-prefix-match lookups and consumes less TCAM space in hardware forwarding tables. This becomes especially important at internet exchange points and transit provider boundaries, where the global BGP table exceeds one million routes. ISPs routinely apply prefix-list filters that accept only aggregates from customers, rejecting more specific announcements to keep the global table manageable.

For enterprise engineers, supernetting improves OSPF and EIGRP convergence by summarising at area or autonomous system boundary routers. When a specific subnet within an aggregate flaps, the summarising router hides the flap from upstream devices, preventing route recomputation across the entire network. This technique, known as route dampening at the summarisation point, is a standard design pattern in large campus and data center networks running link-state protocols.

For example, the subnets 192.168.0.0/24, 192.168.1.0/24, 192.168.2.0/24, and 192.168.3.0/24 can be aggregated into a single supernet: 192.168.0.0/22. Instead of advertising four separate routes, a router can advertise just one, reducing the size of routing tables and improving network performance.

When to Use Supernetting

Supernetting is widely used in BGP route aggregation on the internet, where ISPs summarize customer prefixes into larger blocks. It is also used within enterprise networks for OSPF and EIGRP route summarization at area or autonomous system boundaries. Proper summarization reduces CPU load on routers, speeds up convergence, and improves network stability by hiding flapping subnets.

The calculator finds the smallest supernet by identifying the common prefix bits across all input CIDRs. The result includes the aggregate CIDR and its combined address range. Use this alongside the Subnet Calculator to verify each constituent subnet, and check for conflicts with the Overlap Detector.