dns01.cside.jp has one IP number , which also has a corresponding reverse pointer. or.tv, gll.jp, cssv.jp, jpa.org, html.tv and at least 100 other hosts use this as a nameserver. cside-2nd.com use this as a mailserver. root.dns01.cside.jp are subdomains to this hostname. cside.jp is delegated to two nameservers, however one delegated nameserver is missing in the zone. All nameservers are at cside.jp themselves. They are on the same IP network. Incoming mail for cside.jp is handled by one mailserver also at cside.jp. cside.jp has one IP number. dns01.cside.jp is hosted on a server in Japan.