Route Summarization: Subnetting – Part 4

In the final part of this series, we are going to cover Route Summarization. If you haven’t read the other parts of the series then we covered Standard Length Subnet Masks (SLSM), how to find all the subnets in a given network and prefix, and then we moved on to Variable Length Subnet Masks (VLSM). They are always worth a bit of a recap at any stage.

Route summarization is very similar to finding all the subnets within a network and a prefix. In fact, much of the logic behind it is exactly the same.

Again we can do this through the binary method (which I actually think is easier for this) or the decimal method.

Route Summarization: Binary Method

In the binary method we follow this process:

1. Write down the binary version of each subnet
2. Look to see where the numbers start to differ and draw a line down that portion.
3. Copy the bits that are the same into a new line and write 0s for the remaining bits. 
4. Convert the result from step 3 back into decimal.

We could use the same subnets as we used in part 2 of this series, but that would be a little too easy, so lets use (as we have done so far) the example from the Odom book and use the subnets of 172.31.20.0, 172.31.21.0, 172.31.22.0, and 172.31.23.0 with a /24 prefix.

Subnet Octet 1 Octet 2 Octet 3 Octet 4
172.31.20.0/24 10101100 00011111 00010100 00000000
172.31.21.0/24 10101100 00011111 00010101 00000000
172.31.22.0/24 10101100 00011111 00010110 00000000
172.31.23.0/24 10101100 00011111 00010111 00000000
Summary 10101100 00011111 00010100 00000000

Now we convert the summary address back into decimal and we have a prefix length of /22 and an address of 172.31.20.0

Route Summarization: Decimal Method

In the decimal method, we do the following:

1. Count the number of subnets, then find the smallest value of y where 2y equals the number of subnets. So for four subnets 22 = 4, therefore y = 2
2. Subtract Y from the longest prefix length of the component subnets (with /24 subnets this would be 24 – 2 = 22)
3. Taking the lowest numeric subnet number in the component subnets, and then using it as an IP address calculate the subnet that this would live in (i.e. 172.31.20.0/22).
4. Repeat step 3 this time using the largest component subnet. If the resulting subnet matches step 3 then this is the best-summarized route. (172.31.23.0 with a /22 gets us to 172.31.20.0/22)
5. If step 4 does not get the best result repeat steps 3 and 4 again, this time subtracting 1 from the previously used prefix length.

i do think that the binary method is clearly easier than the decimal method here, but then everyone is different and you may find that the decimal method works better for you.

And that wraps up this series on subnetting. Again I will be posting more examples in the near future.