[AusNOG] IPv4 - Free /48 from APNIC

Karl Auer kauer at biplane.com.au
Tue Mar 5 14:47:57 EST 2013

On Tue, 2013-03-05 at 11:10 +0800, Damian Guppy wrote:
> In your example the IPv4 prefix would be 192.168.1 assuming it is a /24
> subnet.

We apply human common sense to come up with that assumption (and it's
probably right) but technically that address could be in any of many
different prefixes.

> In most
> setups in IPv6 the host part of the address is determined by the devices
> MAC address plus a couple extra bits in the middle

This is just one way for hosts to get addresses. They can generate them
using this algorithm, or choose random 64-bit interface IDs, or use
DHCPv6 to obtain addresses, or get addresses statically configured... or
all of these, at the same time.

> , and there are multiple
> prefixes assigned by the router/network, as well as a 'local' prefix.

There can be multiple prefixes advertised by the router(s), or there may
be only one, or there may be none at all. The "local" prefix, by which I
guess you mean the "link local" prefix, does not involve any routers,
it's built into the IPv6 stack.

> In the end you will often have 3-4 IPv6 addresses per device in this sort of
> multi homing setup

It's not multi-homing, it's multi-prefix. Multi-homing is something else
entirely, But you may well have lots of addresses on one interface.

> , all with the same host portion but different prefix
> portions.

Not necessarily - they could be different "host portions" (interface
IDs) and /or different prefixes, depending on the mechanisms used to
generate or obtain the addresses.

IPv6 is a wonderful world of mixing and matching; you can have it all,
and all at the same time :-)

Regards, K.

Karl Auer (kauer at biplane.com.au)

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