CIDR Summarizer

Find the smallest supernet that covers a list of IPv4 networks. Detects address gaps, calculates utilisation efficiency, and generates Cisco IOS summary route commands for EIGRP and OSPF.

What is route summarization?

Route summarization (also called route aggregation or supernetting) replaces multiple specific routes with a single summary route that covers all of them. The benefit is a smaller routing table, less routing protocol overhead, and faster convergence — each router processes fewer routes and fewer updates.

The summary route is advertised by the summarizing router. Downstream routers see only the summary and do not need to know about the individual component networks behind it.

How summarization is calculated

The algorithm finds the shortest prefix that covers the entire range of addresses in the input networks:

  1. Sort all input networks by their network address.
  2. Find the lowest network address and the highest broadcast address across all inputs.
  3. Find the smallest power-of-2 block that spans from the lowest to the highest address.
  4. Align the block to a boundary of its own size (the network address must be a multiple of the block size).

Example: summarizing 10.1.0.0/24, 10.1.1.0/24, 10.1.2.0/24, 10.1.3.0/24.

Address gaps

If the input networks do not fill the entire supernet contiguously, the summary route covers addresses that are not in any component network. This is usually acceptable — traffic to gap addresses will be routed to the summarizing router, which will drop it (no more-specific route exists) or forward it via a default route.

However, if another network legitimately uses those gap addresses elsewhere in your routing domain, advertising the summary will attract traffic that should go elsewhere. Always check the efficiency percentage and gap report before advertising a summary route.

EIGRP route summarization

EIGRP summarization is configured per interface. The command advertises the summary out the specified interface instead of the component routes:

interface GigabitEthernet0/0
 ip summary-address eigrp 100 10.1.0.0 255.255.252.0

EIGRP automatically installs a Null0 route for the summary to prevent routing loops when no more-specific route exists. Verify with show ip route 10.1.0.0 — you should see both the summary route and the Null0 entry.

OSPF inter-area summarization

OSPF summarization is configured on the Area Border Router (ABR), which sits between two OSPF areas. The area range command summarizes LSAs going from one area into another:

router ospf 1
 area 1 range 10.1.0.0 255.255.252.0

This summarizes all networks in area 1 that fall within 10.1.0.0/22 into a single Type 3 Summary LSA advertised into the backbone (area 0). OSPF does not support summarization within a single area — only at area boundaries.