Route Summarization

1. Introduction

Route Summarization (also called route aggregation or supernetting) allows a routing device to replace multiple specific routes with a single summary route that represents all of them.

Normally, when a router learns several different networks, each of them appears as a separate entry in the routing table. That’s fine if you only have a few routes. But what if your network has hundreds or thousands of subnets?

Router R1 with three separate static routes to networks 172.18.20.0, 172.18.21.0 and 172.18.22.0 via R2

Figure 1 – Topology before route summarization

In that case, the routing table becomes very large. Every time a packet enters the router, it must check all those entries before making a routing decision. This wastes memory, increases CPU usage, and makes routing lookups slower.

To solve this, route summarization combines multiple routes into a single larger summary route.

Router R1 using a single summary route 172.18.20.0/22 to reach multiple networks through R2

Figure 2 – Topology after route summarization

Here, you can clearly see the difference:

  • In Figure 1, R1 has three static routes, one for each network behind R2 (172.18.20.0/24, 172.18.21.0/24, 172.18.22.0/24).

  • In Figure 2, R1 uses just one summary route (172.18.20.0/22) that covers all three networks.

This technique is heavily used on the Internet. Routers must handle routes to every public IP address, and without summarization the global routing tables would grow too large for devices to store and process efficiently.

By summarizing networks into larger blocks, routers reduce the number of entries in their routing tables, saving both CPU and memory.

2. Without Summarization

In our example, imagine that R1 needs to learn about the following networks located behind R2:

  • 172.18.20.0/24

  • 172.18.21.0/24

  • 172.18.22.0/24

network topology with R1 and R2 connected to three /24 networks before route summarization

Figure 3 – Topology used for route summarization

Without summarization, we would need to configure one static route for each individual network (or learn them separately via a dynamic routing protocol).

In this case, we use static routes and set the next-hop IP address of R2. That means three separate commands:

R1(config)# ip route 172.18.20.0 255.255.255.0 192.168.1.2
R1(config)# ip route 172.18.21.0 255.255.255.0 192.168.1.2
R1(config)# ip route 172.18.22.0 255.255.255.0 192.168.1.2

If we now check the routing table, we can clearly see one entry per network:

R1# show ip route
Gateway of last resort is not set

     172.18.0.0/16 is variably subnetted, 4 subnets, 2 masks
S       172.18.20.0/22 [1/0] via 192.168.1.2
S       172.18.20.0/24 [1/0] via 192.168.1.2
S       172.18.21.0/24 [1/0] via 192.168.1.2
S       172.18.22.0/24 [1/0] via 192.168.1.2
C    192.168.1.0/24 is directly connected, GigabitEthernet0/0

This configuration works, but it’s not efficient.
All three routes point to the same next hop (192.168.1.2). Instead of keeping multiple separate routes, wouldn’t it be better to use just one route that covers them all?

3. How Summarization Works ?

Since R1 needs to summarize these three networks, we must calculate a bigger network that includes all of them:

  • 172.18.20.0/24

  • 172.18.21.0/24

  • 172.18.22.0/24

route summarization example where the router suggests summarizing three networks into one route

Figure 4 – Router indicating a possible route summarization

In other words, instead of keeping three separate /24 networks, we’ll summarize them into a single larger prefix that represents the whole range.

But this only works when the networks are contiguous, meaning their IP address ranges are directly next to each other with no missing networks in between.

Now, let’s calculate our summary step by step.

Step 1 – Convert them into binary

To find the summary, we first write the networks in binary.
This makes it easier to see how many prefix bits they have in common.

172.18.20.0 = 10101100.00010010.00010100.00000000
172.18.21.0 = 10101100.00010010.00010101.00000000
172.18.22.0 = 10101100.00010010.00010110.00000000

Step 2 – Find the common part

Let’s now compare the addresses bit by bit from left to right.
We want to identify the part of the prefix that is the same across all three networks.
This shared prefix is what defines the summarization.

172.18.20.0 = 10101100.00010010.00010100.00000000
172.18.21.0 = 10101100.00010010.00010101.00000000
172.18.22.0 = 10101100.00010010.00010110.00000000
                                     ↑  until here bits are the same

As you can see, the first 22 bits are identical.
After that, they differ, so the common prefix ends at bit 22.

Step 3 – Build the summary

The number of shared bits gives us the prefix length of the summary.
Here, we found 22 common bits, so the summary prefix is /22.

To get the summary address, we keep these 22 bits and set the rest to 0:

10101100.00010010.00010100.00000000 → 172.18.20.0

  • Summary network: 172.18.20.0

  • Prefix length: /22

  • Subnet mask: 255.255.252.0

The final summary route is: 172.18.20.0/22

Step 4 – Check which networks it covers

A /22 covers 4 consecutive /24 networks.
Why 4? Because with 2 extra network bits, we get 2² = 4 subnets.

That means our summary includes:

  • 172.18.20.0/24

  • 172.18.21.0/24

  • 172.18.22.0/24

  • 172.18.23.0/24 (even if not in use yet)

4. Route Summarization Configuration

On R1, we can now replace the three individual static routes with a single summary route.

First, remove the existing static routes:

R1(config)# no ip route 172.18.20.0 255.255.255.0 192.168.1.2
R1(config)# no ip route 172.18.21.0 255.255.255.0 192.168.1.2
R1(config)# no ip route 172.18.22.0 255.255.255.0 192.168.1.2

Then configure the new summary route:

R1(config)# ip route 172.18.20.0 255.255.252.0 192.168.1.2

If we check the routing table again, you can now see the summarized entry:

R1# show ip route
Gateway of last resort is not set

     172.18.0.0/22 is subnetted, 1 subnets
S       172.18.20.0 [1/0] via 192.168.1.2
C    192.168.1.0/24 is directly connected, GigabitEthernet0/0

Instead of having three separate static routes, R1 now keeps just one summary route that covers all three networks.

Test connectivity

To confirm everything works, we can test by pinging PC1, which belongs to the 172.18.20.0/24 subnet.

R1 ping PC1 by using the route summarization in the routing table

Figure 5 - Ping using the Route Summarization

The router uses the summary route to forward the packet toward R2.
R2, which is directly connected to the 172.18.20.0/24 subnet, then delivers the packet to PC1.

The communication succeeds, proving that the summary route is working correctly.

5. Conclusion

Route summarization is a crucial technique that reduces the size of routing tables and improves network efficiency.

In our lab, we saw how R1 replaced three static routes with a single summary route, while still being able to reach all subnets.

The same principle applies on a global scale: on the Internet, routers use summarization to advertise larger blocks of addresses instead of thousands of small subnets. Without it, global routing tables would be far too large for routers to handle.

Flat-style diagram showing routers inside an Internet cloud using route summarization with /16 and /12 prefixes

Figure 6 – Global Routing Table with Summarization

By summarizing routes:

  • TThe routing table has fewer entries.

  • Routers use less CPU and memory for routing lookups.

  • Routing updates in dynamic routing protocols are smaller and faster.

For the CCNA exam, always remember that summarization works only with contiguous networks and that you must find the common prefix bits to build the correct summary.