IPv6: Address a Link and Route a Prefix

This host's upstream link is IPv6-only, and right now it's down and unaddressed. The router at the other end is already configured. It lives in a network namespace on this box, so you can test from its side too.

/root/case/assignment.env names the interface, the link's /64, the router's address and a remote /64 behind the router (all differ per session; /root/case/brief.txt has the details). On the live system:

1. Give the interface a global IPv6 address inside the link's /64, with prefix length /64. Any host ID works except the router's own ::1. The fe80:: link-local address doesn't count: the kernel adds one to every IPv6 interface by itself.

2. Bring the link up so the router can reach that address.

3. Route the remote /64 via the router, so the host can answer traffic from that network.

Grading pings your address from the router: first from its link address, then from its address on the remote network. So it proves real two-way reachability, and any valid route form passes (via the router's global or link-local address). Nothing needs to survive a reboot.