IPV6 on a LAN

[#1]

In thinking about the switch to IPv6, one question I keep coming up against is how does the local LAN work. I have about eight devices on my LAN ( printers, NASs, and a couple of tablets as well as the PC).

As I understand things the IPv6 address has a 64 bit prefix and a 64 bit device identifier - generated from the MAC address of the device.

With an IPv6 capable ISP, the prefix is supplied by the ISP as part of the router's IP address, and the rest of the LAN then constructs its IP addresses from that prefix and the local device MAC.

That implies that unless the ISP is providing static IP addresses (do all IPv6 capable ISP's do that?) then every time the router restarts and gets a new IP address, all the internal  LAN IP addresses change as well. How do those on the trial cope with this? Do you need a router that acts as an IPV6 DNS server and address all the devices on the LAN by name, or is there some other trick used to allow internal LANs' to continue to function?

Do Plusnet provide static IPv6 addresses on the trial? If not how do those on the trial handle LAN addressing?

[#2]

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Hi Wisty,

@wisty wrote:

As I understand things the IPv6 address has a 64 bit prefix and a 64 bit device identifier - generated from the MAC address of the device.

That is one way of configuring an address on a device (known as Stateless Address Autoconfiguration) however it is worth knowing that there are others including DHCPv6 and static (manually configured) addressing.

With an IPv6 capable ISP, the prefix is supplied by the ISP as part of the router's IP address, and the rest of the LAN then constructs its IP addresses from that prefix and the local device MAC.

The ISP will typically delegate a prefix (such as a /56 in Plusnet's case) to the router which will, in turn, assign a /64 prefix to the LAN. This prefix is advertised to the LAN through a Router Advertisement (RA) and from this devices will configure themselves with an address as you describe.

That implies that unless the ISP is providing static IP addresses (do all IPv6 capable ISP's do that?)

Some provide a static prefix, some (most?) seem to be dynamic (albeit with varying levels of dynamism).

then every time the router restarts and gets a new IP address, all the internal  LAN IP addresses change as well.

If the router receives a new prefix (note it is not a single address) then yes, all LAN IP addresses constructed from the old prefix will need to change. The LAN is informed of the new prefix with a new RA.

How do those on the trial cope with this?

It can be a bit of a pain to be honest. Clients won't suffer as they'll just pick up a the new prefix, construct a new address with it, and just carrying on without issue. However, those with more of a server role can be severely impacted by it, particularly those that are configured with manually-constructed addresses and/or those that have DNS records pointing to them.

Do you need a router that acts as an IPV6 DNS server and address all the devices on the LAN by name, or is there some other trick used to allow internal LANs' to continue to function?

In my case I have a script running on a server that detects that the prefix has changed and automatically reconfigures itself with a new address and updates DNS to suit.

Note however that in addition to globally unique addresses (that Plusnet provides) all LAN devices will have a locally unique address and this will remain the same regardless what happens at the ISP. These addresses can be used to provide stability on the LAN for those that may otherwise be affected if the ISPs addresses e.g. printers and other 'server like' devices, particularly those that don't need or don't want to be reached from the Internet.

There are other mechanisms available e.g. network prefix translation (similar to NAT in IPv4 yet it works on the prefix only and hence doesn't suffer the same problems as NAT) however many such techniques are still experimental and not widely deployed.

[#3]

Yes I run stateless ipv6 on my router - its usually run through the radvd process, which lukily is what is implimented on my router. The router gets it IPv6 address by 6in4, so although it has an IPv6 address it goes out onto the wan in IPv4 - so works on plusnet, and plusnet also routers the 6in4 packets correctly.

Now back to the LAN, and radvd - it hands out stateless IPv6 adresses to devices on the nework. Its done this smoothly, and through changes in the WAN connection. All machines have stayed up, and ipv6 adressing works through a change in ipv6 address.

So I expect my set up would work find, if switched to ipv6 wan, I'd not longer need to use 6in4 - but the radvd process work work just the same and handle the lan stateless adressing.

[#4]

While it's probably a bit late now ...

For LAN only use, you can use link local addresses https://en.wikipedia.org/wiki/Link-local_address which you'll find on every IPv6 enabled interface by default. These won't change unless you change the interface. But you cannot route them so they don't work if you want more than one network on your site.

Or, you can generate a ULA prefix for your site and use that. https://en.wikipedia.org/wiki/Unique_local_address That would give you a /48 allowing you to have up to 65536 /64 networks in your site.