On Tue, Sep 20, 2011 at 09:34:46AM -0700, Stig Venaas wrote:
> On 9/20/2011 2:03 AM, Iljitsch van Beijnum wrote:
> >On 20 sep 2011, at 9:58, sth...@nethelp.no wrote:
> >
> >>For pt2pt SDH links you want /127 to avoid the ping-pong problem,
> >>not /126.
> >
> >That's nice (as long as your routers ignore the all zeros anycast address),
> >but the question was: how do we write down IPv6 prefixes? Are we ok with
> >simply leaving off the undefined part, or do we insist on proper inclusion
> >of the zeros? If the former, how do we avoid ambiguity for prefixes such as
> >::ffff/96?
>
> I agree it is ambiguous in some sense. That is, I expect any reasonable
> implementation to treat ::ffff/96 as ending in ffff, and being
> equivalent to ::/96.
mostly...
> However, for us humans, why would you write ::ffff/96 and not ::/96?
Because I want to specify the address of an interface and its on-link
network space at the same time.
> I think one can pretty much expect that the bits beyond the prefix length
> are 0 when writing prefixes.
> I don't see a big deal with using ::ffff/96 as a sloppy notation on
> whiteboards etc.
For the reason I gave, I don't think so. I don't want a sloppy notation
to be in use to force me to context-sensitive parsing of addresses; this
will lead to mistakes, when the context is lost. Especially probable with
whiteboard notation which will copied manually, maybe only partially...
> But for documents like Internet Drafts and RFCs, we
> should probably make sure to write the trailing 0s.
With this I agree.
> I myself having got some pushback for writing e.g. 239/8. Even though
> it is not ambiguous.
Well, but the shortened notation for IPv4 networks has its roots in the
classful and pre-classful IPv4 notation, and is justified from there at
least to some degree... we don't have that... luxury... with v6. On the
other hand, we did specify a syntax for shortening (compression of zeros)
for IPv6, so let's stick to that, even in our whiteboard notes.
Regards,
-is
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