[Bradford] IPv6

[David Spencer]

On Tue, 7 May 2019, 18:07 Devo Too via Bradford, <
bradford at mailman.lug.org.uk> wrote:

> Hi Folks,
>
> I'm moving slowly into 21st century IT. Am I correct in thinking this
> 'fe80::207:cbff:fe04:3b5' is the IPv6 address of a server, with the /64
> giving the subnet?
>
>   inet6 fe80::207:cbff:fe04:3b5/64 scope link
>         valid_lft forever preferred_lft forever
>
> It's from the eth0 output of the command 'ip -6 addr show'
>
> Thanks,
>
> Mike
>
> --
> Bradford mailing list
> Bradford at mailman.lug.org.uk
> https://mailman.lug.org.uk/mailman/listinfo/bradford


Hi Mike,

It's the *link local* address of the *interface* on the server. Link local
addresses are bugger all use, they're mostly used to bootstrap real
addresses.

In more detail -- anything that starts 'fe80' is a 'link local' address,
not a proper ipv6 address. It's the ipv6 equivalent of those awful 169.254
ipv4 addresses you get in lonely un-networked printers or windows boxes. In
the world of ipv6 every interface that's up will have a unique fe80 address
based on the mac address. But to do anything useful you'll need an upstream
connection of some sort, which will end up giving the interface one or more
additional proper global routeable addresses. Proper global routeable
addresses start with 2 :)

There are multiple ways of getting a proper globally routeable address, the
easiest and most common is to have your interface get it from the upstream
router.

This needs
 (1) the router to have an ipv6 address, and
 (2) the router to have Router Advertisements enabled, and
 (3) the interface to be plugged in and to have stateless autoconfiguration
(SLAAC) enabled.

About 1, BT and Sky dish out ipv6 ranges ("prefixes") to customer routers,
so long as the router is up to the job. If your ISP doesn't do ipv6 you
will have to work round them by setting up a free tunnel, or vpn, or wait
for them to get with the programme (Virgin) or give up and dump the tossers
(Talktalk).

About 2, this is almost certainly enabled by default if the router is ipv6
aware.

About 3, this is enabled on pretty much every Linux distro. SLAAC works a
bit like DHCP but better (dhcpv6 is a thing too, but is quite different to
dhcp, and is mostly poo and not used). You can see it happening (or not
happening) in 'dmesg' when interfaces come up.

"/64" tells the network stack how much of the address is in the local
subnet, and therefore what's on the lan or the wan when you talk to another
address. This works just like ipv4 subnetting, but the notation is used
more ubiquitously so that you'll never need to see the equivalent of the
ipv4 .255.255 netmask bollocks. Commonly your ISP actually gives you a /56
prefix, and so you will theoretically have 256 /64 networks of
18,446,744,073,709,551,616 hosts each to play with, but your typical
domestic router will just use the first /64 network in the range and ignore
the rest. Nobody ever subdivides a /64 network.

There are other setups (tunnels, ULA addresses) but you don't want to get
into all that unless there's some reason why you can't have a bog standard
ISP-and-router setup.

Lots of good writeups out there describe things like link-local addresses
and slaac and router advertisements in more detail, but of course you can't
look them up until you know the right terminology, so hopefully I've just
provided that :) ... Wikipedia is generally ok on these topics.

Cheers
-D.
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