IP aggregation aka Supernetting aka Summarization is used to summarize a number of subnets into one big supernet. It serves as a critical tool to reduce the size of routing table and helps reducing the processing power and memory usage of the router. Moreover, these days routers are built of very high cpu and allocated a high memory space, hence I don’t
consider cpu and memory as a good factor for using aggregation. Aggregation can be lethal in reducing the size of routing table which may become a big stability factor in use of link state routing protocols like ospf and ISIS.
People have different methods to aggregate the subnets. I have my own method which is quick, easy to understand and accurate. I would recommend you to follow these simple steps and do aggregation in seconds rather than going through that lengthy bit by bit process described in most of the text books.
My recommended process for aggregation is as follows:
Always remember following reference table:
| 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
Example: We have to summarize the following subnet
10.23.12.0/24, 10.23.13.0/24, 10.23.14.0/24……10.23.78.0/24
Step 1: Identify the last network octet of first subnet. Here network octets are 10.23.12 hence in this example last octet of first network becomes 12.
Step 1: Check the number, If it is even then it can be network address of the summarized subnet. If it is an odd number it can’t be network address of summarized address. Here the number is 12 in 10.23.12.0/24 means it can be the network address of the summarized subnet.
Step 2: Divide the number by the numbers written in the upper half of the table one by one.
Step 3: It is divisible by 2 and 4. These numbers in upper half of table are the number of subnets which can be aggregated in this range. Since it is divisible by only 2 and 4, it can be aggregated in a summary address which is having either 2 subnets or 4. We can summarize the maximum here. In this case 4.
Step 4: Subtract number corresponding to lower half of the table from /mask value. So here 24-2 = 22. Hence first aggregated IP is 10.23.12.0/22 having 10.23.12.0/24, 10.23.13.0/24, 10.23.14.0/24 and 10.23.15.0/24 summarized in it.
Repeat the above four steps until you reach the final IP subnet which is 10.23.78.0/24 and final result should be:
| 10.23.12.0/22 |
| 10.23.16.0/20 |
| 10.23.32.0/19 |
| 10.23.64.0/21 |
| 10.23.72.0/22 |
| 10.23.76.0/23 |
| 10.23.78.0/24 |
Hence with the help of summarization we have reduced 67 subnets to 7 subnets.
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