ICS on Windows 10 host with IPv6

Question

My ethernet interface is connected to the internet (dual stack, has both IPv4 and IPv6), and I want to use ICS to share the internet connection to another interface.

When enabled, my non-internet iface gets an DHCPv4 address in the 192.168.137.0/24 subnet, and can reach the internet via the internet-connected iface as gateway (192.168.137.1).

However, the non-internet iface does not get any IPv6 address: Neither a DHCPv6, nor a SLAAC.

Am I setting up something wrong, or is ICS simply not working for IPv6?

Answer 1

There are a few ways to share IPv6, and AFAIK none of them are implemented by Windows:

1. Obtain a new global /64 prefix through DHCPv6-PD and do regular IPv6 routing. This is how your main router obtains its current LAN prefix – but many home routers don't support "nested" prefix delegation, so your PC usually cannot ask your main router to get a dedicated prefix for itself.

2. Assign a private prefix and do IPv6 NAT, just like it's done with IPv4. As far as I know, Windows does not implement IPv6 NAT (which is why WSL2 doesn't have IPv6), and really it would kind-of defeat the purpose of IPv6 anyway.

3. Duplicate the "Internet side" prefix and do "NDP proxying" (the IPv6 equivalent of proxy-ARP). As far as I know, Windows doesn't implement this either. It would technically work but it's still definitely a hack – same result as if you had just bridged the interfaces together, but with more hacks to make it work.

(That is to say, Windows does support pure IPv6 routing – I've used a WinXP PC as my main IPv6 router in the past – it's the need for a dedicated /64 that is the main problem here.)

So although I can't be 100% sure that ICS is IPv4-only (though I'm 90% sure on the grounds of WSL being IPv4-only), I would still suggest just bridging the interfaces together (i.e. turning your PC into an Ethernet switch) – the option is still there in ncpa.cpl when you select two or more interfaces.