Going to other planets would require a total re-architecting of our communications infrastructure anyway. There’s such distance too it’s not really viable to have a shared internet. Even Mars would have up to 22 minute latency at peak. So I don’t think it makes sense to plan our current internet around potential future space colonization.

Even so, IPv6 is truly massive. We could give a /64 to every square centimeter of the Earth’s surface and still have IPs to spare. Frankly, I think the protocol itself will be obsolete before we run out.