ns-tk021.ocn.ad.jp has one IP number , which also has a corresponding reverse pointer. my-j.net, aeba.net, m5588.net, prtec.net, kao.co.jp and at least 100 other hosts use this as a nameserver. ns1.ns-tk021.ocn.ad.jp are subdomains to this hostname. ocn.ad.jp is a domain controlled by two nameservers at ocn.ad.jp themselves. They are on different IP networks. Incoming mail for ocn.ad.jp is handled by two mailservers also at ocn.ad.jp. They are on the same IP network. ocn.ad.jp has two IP numbers. They are on the same IP network. ns-tk021.ocn.ad.jp is hosted on a server in Japan.