This tutorial shows how you can enable networking in Xen guests (domU) on Hetzner’s DS servers. With the DS servers, you can get a subnet of eight additional IPs (or more) – usually that subnet is different from the subnet that the server’s main IP is from. More here
The problem is that these additional IPs are bound to the MAC address of the host system (dom0) – Hetzner’s routers will dump IP packets if they come from an unknown MAC address. This means we cannot use Xen’s bridged mode, but must switch to Xen’s routed mode where the host system (dom0) acts as the gateway for the guests.