IPv6 Summarization Qs

[Muhammed Midlaj]

Hello All,

From the Book 'CCIE and CCNP enterprise core ENCOR 350-401' page 234, There is a statement as follows:

"NOTE A common mistake with summarization of IPv6 addresses is to confuse hex with
decimal. We typically perform summarization logic in decimal, and the first and third
digits in an octet should not be confused as decimal values. For example, the IPv6 address
2001::1/128 is not 20 and 1 in decimal format. The number 2001::1/128 is 32 and 1."

I could not exactly understood what the author mentioned here in the last sentence, Can someone explain it to me if possible please?

Also in the book, the same chapter, the Author summarized the following networks  

2001:db8:0::1/128,

2001:db8:0::2/128 and

2001:db8:0::3/128

as 'area 0 range 2001:db8:0:0::/65' 

But configuring like 'area 0 range 2001:db8:0:0::/127' is also possible right? is there any wrong with it?

I am bit confused since it is regarding IPv6. 

Thanks in advance.

Warm Regards

[Jose Quesada]

Hi Muhammed,

What the book explains is that when you are seeing “20”, that you need to be aware that it is a HEX number and not a decimal. If you take that 20 and change it to Decimal, it is 32.

The same logic when the autor on the book says that it needs to summarize the IPv6 address with the “12” at the end, it is indeed 18 in decimal value, and therefore, you will need 5 bits to represent 18 in binary, hence, you need 5 bits to summarize it, and that is why the summary address has a mask of 59.


Regarding your question about the best summary for 2001:db8:0::1/128, 2001:db8:0::2/128 and 2001:db8:0::3/128, it is indeed 2001:db8::/62. This based on the fact that 1,2,3 and will take two bits in total.

I was not able see on the book, the /65 summary that you mentioned.
Regards,

Jose.

[Muhammed Midlaj]

Hi @Jose Quesada ,

I am sorry, I still did not understood regarding the same

I had a typo regarding the page number, Now I've corrected it, it is in pg.234.

Warm Regards

[Harold Ritter · Accepted solution]

Hi @Muhammed Midlaj ,

> "NOTE A common mistake with summarization of IPv6 addresses is to confuse hex with
> decimal. We typically perform summarization logic in decimal, and the first and third
> digits in an octet should not be confused as decimal values. For example, the IPv6 address
> 2001::1/128 is not 20 and 1 in decimal format. The number 2001::1/128 is 32 and 1."

With this note the author simply reminds us that the IPv6 addresses are expressed in hexadecimal rather than decimal. This is an important. difference when it comes to summarization.

For instance if you would like to summarize 16 /128 prefixes

2001:db8::/124 summarizes 2001:db8::/128 to 2001:db8::f/128 rather than

2001:db8::/128 to 2001:db8::16/128

> as 'area 0 range 2001:db8:0:0::/65' 

This would definitely summarize 2001:db8::1/128, 2001:db8::2/128 and 2001:db8::3/128, but also a lot more.

> But configuring like 'area 0 range 2001:db8:0:0::/127' is also possible right? is there any wrong with it?

2001:db8:0:0::/127 (or more simply 2001:db8::/127) would only summarize 2001:db8::/128 and 2001:db8::1/128. If you wanted to summarize 2001:db8::1/128, 2001:db8::2/128 and 2001:db8::3/128, you would need to configure at least a /126 as follow:

area 0 range 2001:db8::/126

Regards,

[Muhammed Midlaj]

Thank you @Harold Ritter 

I was thinking too deep and confused. Now I got the clear picture.  

[ErikRevaj6335]

 I literally needed to search for this sentence to understand it. This book is really bad