Why does IPV6 link local address advertised as /64?

Why does IPV6 link local address advertised as /64?

[Stephen Hemminger]

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?

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

[Matti Aarnio]

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

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

[Rémi Denis-Courmont]

On Wednesday 12 November 2008 06:49:06 ext Stephen Hemminger, you 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?

For the same reason that IPv4 link-local (169.254/16) addresses are /16 rather 
than a /32. There are LINK-local addresses, not HOST-local addresses - you 
must be able to talk to other nodes on the link with them.

-- 
Rémi Denis-Courmont
Maemo Software, Nokia Devices R&D