zero suppression vs compression in addresses

Jay Hennigan jay at
Mon May 18 19:30:49 CEST 2009

michael.dillon at wrote:

> So, this draft *DOES* change what is considered to be
> a syntactically correct when it allows "::" to be used
> twice. Admittedly, section 4.2.3 is not very clear and
> it may not survive future discussion, but the bulk of
> this draft is very clear and well-reasoned. It is worth
> reading by anyone who is working on code that converts
> between IPv6 binary and text representations. Regardless
> of its status, the recommendation to output text addresses
> using only lowercase abcdef is a good one.

That isn't how I read the draft.  It does not allow :: to be used twice. 
  Rather, it addresses the ambiguity for those addresses where there are 
more than one "zero" fields separated by a non-zero field.  Such 
addresses have two different places where a :: could be used.  Hence the 
title of 4.2.3 of "When '::' Can Be Used Twice".  It *can't* be used 
twice, but there are some addresses where there are two opportunities to 
use it once.  4.2.3 defines which *one* of those opportunities should be 

The title of 4.2.3 is poorly written, IMHO.  It should be something like 
"Precedence of '::' in cases where there are multiple "0" fields".

Jay Hennigan - CCIE #7880 - Network Engineering - jay at
Impulse Internet Service  -
Your local telephone and internet company - 805 884-6323 - WB6RDV

More information about the ipv6-ops mailing list