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.

What is route summarization and why it matters

Route summarization — also called route aggregation or supernetting — is one of the most important scaling techniques in IP network design. As networks grow, routers accumulate routing table entries for every subnet in the network. Without summarization, a large enterprise might have thousands of entries in its routing table, requiring more memory, more CPU time to process updates, and longer convergence times after a topology change. Summarization collapses multiple specific routes into a single aggregate route, dramatically reducing the size of the routing table and the volume of routing protocol updates.

The key principle: a summarizing router advertises only the aggregate to its neighbors. The neighbors do not need to know about the individual components — they just send traffic toward the summarizing router, which has the full topology behind it. This creates a natural information boundary: internal topology changes (a host moving from one /26 to another within the same /22) do not propagate beyond the summarizing router. The result is faster convergence, less routing protocol traffic, and simpler troubleshooting at the aggregate level. Summarization is most powerful when address space is allocated hierarchically — each site gets a contiguous block, each region gets a contiguous block of site blocks, so each layer in the hierarchy can summarize everything below it.

Route summarization in OSPF and EIGRP — configuration and behavior

OSPF and EIGRP implement summarization differently, and understanding these differences is critical for correct configuration. OSPF summarization is inter-area only: it is configured on the Area Border Router (ABR) using the area range command and applies only to routes crossing from one area into another. You cannot summarize within a single OSPF area. When the area range command is active, the ABR suppresses the individual Type 3 Summary LSAs for the component networks and replaces them with a single Type 3 LSA for the aggregate. If any component network is up, the aggregate is advertised. If all components go down, the aggregate is withdrawn.

EIGRP summarization is per-interface and can occur anywhere in the topology — not just at area boundaries. The command ip summary-address eigrp 100 10.1.0.0 255.255.252.0 on an interface causes the router to advertise only the /22 aggregate out that interface, suppressing the individual component routes. Critically, EIGRP automatically installs a Null0 route for the summary address in the local routing table, which prevents routing loops: if a packet arrives for an address in the aggregate but no more-specific route exists, the Null0 route drops it immediately rather than forwarding it to the default gateway. Always verify the Null0 entry with show ip route 10.1.0.0 after configuring EIGRP summarization.

Benefits of a well-summarized routing table

A compact, well-summarized routing table delivers measurable operational benefits. Convergence time improves because each routing protocol update affects fewer entries — a link failure in area 1 triggers a recalculation only for routes within area 1, not for every prefix in the entire network. Memory consumption drops proportionally with the number of summarized entries; on large networks, this difference can be significant enough to allow older routers to remain in service longer. CPU utilisation during SPF calculations or DUAL computations decreases because fewer destinations are evaluated per topology change.

From an operational standpoint, a clean routing table is dramatically easier to interpret during troubleshooting. When a route is missing, engineers can quickly determine whether the aggregate is missing (upstream problem) or just the specific /24 within it (local problem). This hierarchical structure also makes capacity planning more straightforward: by inspecting the aggregate prefixes at each boundary router, you can immediately see which regions are consuming address space and which have room to grow, without parsing thousands of individual host routes.