Lesson Contents
Summarizing IPv6 prefixes is similar to IPv4 summarization, the big difference is that IPv6 uses 128 bit addresses compared to 32 bits for IPv4 and IPv6 uses hexadecimal addresses.
In this lesson, I’ll explain how to create IPv6 summaries and we’ll walk through some examples together.
Example 1
Let’s start with a simple example:
- 2001:DB8:1234:ABA2::/64
- 2001:DB8:1234:ABC3::/64
Let’s say we have to create a summary that includes the two prefixes above. Each hextet represents 16 bits. The first three hextets are the same (2001:DB8:1234) so we have 16 + 16 + 16 = 48 bits that are the same so far. To find the other bits that are the same we only have to focus on the last hextet:
- ABA2
- ABC3
We’ll have to convert these from hexadecimal to binary to see how many bits are the same:
| ABA2 | 1010101110100010 |
| ABC3 | 1010101111000011 |
I highlighted the bits in red that are the same, the first 9 bits. The remaining blue bits are different. To get our summary address, we have to zero out the blue bits:
| AB80 | 1010101110000000 |
When we calculate this from binary back to hexadecimal we get AB80. The first three hextets are the same and in the 4th octet we have 9 bits that are the same. 48 + 9 = 57 bits. Our summary address will be:
2001:DB8:1234:AB80::/57
That’s how you can create a summary address for IPv6.
Example 2
This time we have the following 3 prefixes:
- 2001:DB8:0:1::/64
- 2001:DB8:0:2::/64
- 2001:DB8:0:3::/64
And our goal is to create the most optimal summary address. The first three hextets are the same so that’s 16 + 16 + 16 = 48 bits that these prefixes have in common. For the remaining bits, we’ll have to look at the 4th hextet in binary:
| 0001 | 0000000000000001 |
| 0002 | 0000000000000010 |
| 0003 | 0000000000000011 |
Keep in mind that each hextet represents 16 bits. The first 14 bits are the same, to get the summary address we have to zero out the blue bits:
| 0000 | 0000000000000000 |
When we calculate this from binary back to hexadecimal we get 0000. The first three hextets are the same and in the 4th octet we have 14 bits that are the same. 48 + 14 = 62 bits. Our summary address will be:
OMG I cannot believe how easy IPV6 is. Granted this is my first time passing through these lessons and if I didn’t recover or think about what I just learned it could easily be forgotten but you really have me looking at it differently.
I like to count backwards from right to left when doing my summaries and one thing about hex that makes it easy is that its in blocks of 4 “0000” so its always “8 4 2 1” those are easy numbers to work with and that part is just simple math but more pattern and grouping really and I love patterns and groupings its how my m
... Continue reading in our forumGlad to hear you like it Max!
It takes time to get used to hex/nibbles. When you just started, it’s easy to forget you are working with hex, not decimal. A common trap is to translate something like 10 (decimal) to 00001010 while it should be 0001 0000 since it’s hexadecimal.
Keep practicing and it will become much easier.
Hello Laz,
I understand that 2001:DB8:1234:AB80::/57 is the “optimal” summarization of 2001:DB8:1234:ABA2::/64 and 2001:DB8:1234:ABC3::/64. but I see this summarization include all the pool [2001:0db8:1234:ab80:0000:0000:0000:0000 - 2001:0db8:1234:abff:ffff:ffff:ffff:ffff] …which must be precised in the documentation
then, if there is no constraint in the exibit asking to exclude all before 2001:db8:1234:ab7f:ffff:ffff:ffff:ffff
... Continue reading in our forumand all after 2001:0db8:1234:ac00:0000:0000:0000:0000, I think an “industrial” summarization may be 2001:DB8:1234:AB00::/56 (pool fr
Hello Laz,
I understand that 2001:DB8:1234:AB80::/57 is the “optimal” summary of 2001:DB8:1234:ABA2::/64 and 2001:DB8:1234:ABC3::/64. but I see that this summary includes the whole pool [2001:0db8:1234:ab80:0000:0000:0000:0000 - 2001:0db8:1234:abff:ffff:ffff:ffff:ffff] … which must be specified in the Lan documentation.
Then, if there is no constraint in the exibit asking to exclude everything before 2001:db8:1234:ab7f:ffff:ffff:ffff:ffff and everything after 2001:0db8:1234:ac00:0000:0000:0000:0000 I thinks that an “industrial” summary can be 2001:DB8:1234:AB00
... Continue reading in our forumHello Walter
Yes, it is true that there are a lot of online tools that will help you summarize and calculate prefixes, and subnet for both IPv4 and IPv6. We will often use those when implementing configurations just to be sure that there is no human error when doing the calculations. However, actually performing them will help us to more deeply understand the concepts and the reasoning behind the implementations, so as a learning exercise, actually doing the calculations is invaluable.
I hope this has been helpful!
Laz