netdev - Re: Why does IPV6 link local address advertised as /64?

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Date:	Wed, 12 Nov 2008 07:29:12 +0200
From:	Matti Aarnio <matti.aarnio@...iler.org>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Why does IPV6 link local address advertised as /64?

On Tue, Nov 11, 2008 at 08:49:06PM -0800, Stephen Hemminger wrote:
> It seems to make more sense to have only the actual host (ie /128)
> on the link-local address or is there some other purpose to
> using a /64?

The purpose of the link-local prefixes is not to report interface
physical address, but enable communication without yet having
full configuration in place  -  this enables ZeroConfig, DHCP6,
etc. mechanisms.

Consider Ethernet with link-local address, you can do peer-to-peer
communication there without global addresses by using MAC address
derived link-local address.  Indeed the global prefix RA's are
link-local multicasts.

Furthermore, with links using _same_ /64 prefix and possibly varying
suffix, all applications using the link-local addresses must also
inform socket binding of the interface where it is to be applied to.

         /Matti Aarnio
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