How to understand the abbreviation of this IPv6 address

JockVSJock

I not understanding how this address is abbreviated.

Long Form: ABCD:0000:0000:0000:0000:0000:0000:0173

to

Short Form: ABCD::0173

How do I know and understand that the two :: represent six quartets?

To me this only represents five quartets.

Alex90

I'm a bit rubbish at explaining stuff but here goes...

You can only can only condense a string of 0000's once is a IPv6 address, for example:

ABCD:0000:0000:1111:0000:0000:0000:0173 would become ABCD:0:0:1111::173 as you condense the longest string of 0's to ::.

It could not be ABCD::1111::173.

What I do is just count up all the individual blocks and then add whatever number required to make that number up to 8.

Another example - there are 5 blocks of numbers in ABCD:0:0:1111::173, 5+3=8 so the :: is equal to 0000.0000.0000

Hope this makes some sort of sense to you.

Cheers,

Alex

Alex90

Also you can removed the leading 0's so in the case of ABCD:0000:0000:0000:0000:0000:0000:0173 you make the last four digits 173 as you remove the leading 0.

You cannot remove trailing 0's so if the address was 1000.1000.1000.1000.1000.1000.1000.1000 is would stay as that because you can't remove the trailing 0's.

markulous

JockVSJock wrote: »

I not understanding how this address is abbreviated.

Long Form: ABCD:0000:0000:0000:0000:0000:0000:0173

to

Short Form: ABCD::0173

How do I know and understand that the two :: represent six quartets?

To me this only represents five quartets.

Alex had a good explanation.

You know there are 8 groups of numbers so if you see ABCD::0173, you know there must be 6 more since you just see ABCD and 0173. Same thing if you saw 3F32::EE21:87A2, you see 3 so therefore the :: represents 5 groups of 0000.

ccnxjr

"Always 8 there are, no more , no less" - Yoda

Which is why you can only use the "::" abbreviation only once.
(hint, if you see "::" appear twice in an address , it's an invalid notation)

JockVSJock

Now it makes sense, thanks.

On top of that, I pulled this example from Test Out's website.

As pointed out, it should be: ABCD::173

mikeybinec

JockVSJock wrote: »

Now it makes sense, thanks.

On top of that, I pulled this example from Test Out's website.

As pointed out, it should be: ABCD::173

Well, yeah, but this would'nt be wrong either: ABCD:0:0:0:0:0:0:0173

It could be posed as a test question!

JockVSJock

mikeybinec wrote: »

Well, yeah, but this would'nt be wrong either: ABCD:0:0:0:0:0:0:0173

Good point...Can we also write this IPv6 address as: ABCD:0:0:0:0:0:0:173

ccnxjr

mikeybinec wrote: »

Well, yeah, but this would'nt be wrong either: ABCD:0:0:0:0:0:0:0173

It could be posed as a test question!

I'm not sure what's in the official cert guide.
However it seems that Leading Zeros , as in the last octect "0173" of

ABCD:0:0:0:0:0:0:0173

In accordance with RFC 5952, section 4.1, leading zeros MUST be suppressed
RFC 5952 - A Recommendation for IPv6 Address Text Representation

Of course, this is fairly new as far as RFCs go.

[h=3]4.1. Handling Leading Zeros in a 16-Bit Field[/h] Leading zeros MUST be suppressed. For example, 2001:0db8::0001 is not acceptable and must be represented as 2001:db8::1. A single 16- bit 0000 field MUST be represented as 0.

A leading zero can potentially be interpreted as an octal representation in some systems.

mikeybinec

JockVSJock wrote: »

Good point...Can we also write this IPv6 address as: ABCD:0:0:0:0:0:0:173

I had a Cisco tutorial that asked to tick off all legitimate representations of
your original IPv6 address. But for "cleanliness", clearly you would lop off all of the zeros and leading zeros

ccnxjr

Thanks for clearing that up!
Was curious.

Ultimately, for certifications, you want to be sure you understand the criteria that the certifying body considers to be correct.
Especially when there are different ways of interpreting a problem.
I'd hate to have cost someone a question based on new and revolutionary RFC's :P