Hello Shivam
For your example above, you can see that the second octet ranges from 16 to 23. This is 8 networks of size /16. To find the summary that will include all of these 8 networks, ask these questions:
- how many /16 networks fit into a summary using /16? Answer: 1
- how many /16 networks fit into a summary using /15? Answer: 2
- how many /16 networks fit into a summary using /14? Answer: 4
- how many /16 networks fit into a summary using /13? Answer: 8
So you have to know first the size of your networks (/16) and how many you want to fit (8). You can do this exercise and find the number.
The network 172.16.0.0/13 has a range of IP addresses of: 172.16.0.0 to 172.23.255.255. This is because, with a prefix of /13, which is a subnet mask of 255.248.0.0:
- the network address is 172.16.0.0
- the first host address is 172.16.0.1
- the last host address is 172.23.255.254
- the broadcast address is 172.23.255.255
You could start from 172.0.0.0 and use a prefix of /11 which would give a range of 172.0.0.0 to 172.31.255.255. That would give us a correct summarization, but it includes many more address ranges than just the ones we want. The goal is always to create the smallest possible summarization range for the addresses in question.
A block size of 8 will give you 0 to 7, but it can also give you 8 to 15, or 16 to 23, or 24 to 31 and so on. The block size does not have to start at 0.
I hope this has been helpful!
Laz