With each Hetzner EQ root server you get a limited number of four IP addresses. Since you need one IP address for your XenServer host system and one for each guest system, the number of guest system is limited by default. To overcome this limitation, you can buy additional IP subnets (/29 with 6 IP addresses up to /24 with 254 IP addresses).
Due to the MAC filters in the Hetzner firewalls (see “Using XenServer in a routed IP network”), traffic from the virtual machines need to be routed by the host system. To enable this routing, I assigned one of the IP addresses in the new subnet to my host system by adding
ip addr add AAA.BBB.CCC.DDD/EE dev xenbr0
at the end of the file /etc/rc.d/rc.local and using this IP address as gateway in all my guest systems with IP addresses in this specific subnet.