When we think about ways to efficiently manage IP networks and their addressing, subnetting is often the first technique that comes to mind. Subnetting is an extremely important topic in modern IP management, but it only focuses on carving assigned network space into smaller and smaller parts. Address summarization, often referred to as supernetting, does the opposite – representing several blocks of IP addresses with a single, comprehensive prefix.
IP Address summarization is useful in a number of ways, but is particularly helpful when manually summarizing routes to reduce routing table entries and overhead. Let’s walk through an example to teach you the steps.
Network Summarization Example
Networks to be summarized:
Step 1. Find the interesting octet
The first step in summarizing a list of networks is to identify the first octet where the decimal value is not the same.
In this example, the first octet for all networks is 172. No difference there. The second octet is 16 for every network in the list. Again, no difference either. The third octet is where we see different values, which is what we’ll call the interesting octet. That is where we need to focus our attention.
Step 2. Convert the interesting octet values to binary
172.16.3.0/24 > 172.16.00000011.0/24
172.16.4.0/24 > 172.16.00000100.0/24
172.16.5.0/24 > 172.16.00000101.0/24
172.16.6.0/24 > 172.16.00000110.0/24
172.16.7.0/24 > 172.16.00000111.0/24
172.16.8.0/24 > 172.16.00001000.0/24
172.16.9.0/24 > 172.16.00001001.0/24
Step 3. Identify the common bits and convert to decimal
The common bits in the third octet are shown in red above. Now we simply need to add trailing zeros to the end and convert it to decimal. This gives us the summary network address.
172.16.00000000.0 > 172.16.0.0 = Summary Address
Step 4. Count the number of leading common bits to find the mask
To find the summary mask, we need to count the number of bits from left to right until they no longer match. We already determined that in our example the first and second octets match, each with 8 bits. The third octet contains four matching bits (in red). Combined, this gives us a mask of 20 (8+8+4).
172.16.00000000.0 > 10101100.00010000.00000000.00000000 (20 leading common bits)
172.16.0.0/20 = Summary Address
This was a simple summary example, but the same process will work for any networks. If you have other IP summary questions or need more examples, feel free to leave a comment below.