ns56.1und1.de has one IP number , which also has a corresponding reverse pointer. ns56.kundenserver.de point to the same IP. swe.de, vnu.de, osy.de, fdc.de, dhs.de and at least 100 other hosts use this as a nameserver. chrystall.de use this as a mailserver. 1und1.de is a domain controlled by two nameservers at schlund.de. They are on the same IP network. Incoming mail for 1und1.de is handled by three mailservers also at schlund.de. All of them are on the same IP network. 1und1.de has one IP number. ns56.1und1.de is hosted on a server in Germany.