CIDR and Network Summarization

Classless Inter-Domain Routing (CIDR) is not just for subnetting; it's also the key to a powerful technique called route summarization (or route aggregation). This process allows routers to combine multiple specific network routes into a single, more general route.

The primary benefit is a smaller routing table. A router with 10 routes is faster and uses less memory than a router with 10,000 routes. Summarization is crucial for the stability and scalability of the entire internet.

How to Summarize Routes

The trick to summarization is finding the common bits that all the network addresses share.

Let's say a router has the following four networks it needs to advertise:

• 192.168.0.0/24
• 192.168.1.0/24
• 192.168.2.0/24
• 192.168.3.0/24

Step 1: Convert to Binary

We only need to focus on the octet that is changing, which is the third octet.

192.168.0.0  -> 11000000.10101000.00000000.00000000
192.168.1.0  -> 11000000.10101000.00000001.00000000
192.168.2.0  -> 11000000.10101000.00000010.00000000
192.168.3.0  -> 11000000.10101000.00000011.00000000

Step 2: Find the Matching Bits

Look from left to right and find how many bits match across all the addresses.

3rd Octet:
00000000
00000001
00000010
00000011

We can see that the first 6 bits of the third octet are the same for all four networks. The 7th bit is where they start to differ.

Step 3: Calculate the Summary Route

The number of matching bits becomes our new summary prefix.

• The first two octets are 16 bits.
• We found 6 matching bits in the third octet.
• Total matching bits: 16 + 6 = 22.
• Our new summary prefix is /22.

The summary route is the first network address in the range with the new prefix.

Summary Route: 192.168.0.0/22

You can verify this with our CIDR Calculator. A /22 network starting at 192.168.0.0 covers the entire range from 192.168.0.0 to 192.168.3.255, perfectly encompassing our four original networks. The router can now advertise this one summary route instead of four individual routes.