RFC 1034 に現れる単語は"zone cuts"と"each zone cut"などから分かるように、 ゾーン分割（という動作）ではなく、ゾーンの「切断点」を指す名詞である。
- "above cuts" なども現れる。
1.1. RFC 1034
The RRs that describe cuts around the bottom of the zone are NS RRs that name the servers for the subzones.
Since the cuts are between nodes, these RRs are NOT part of the authoritative data of the zone, and should be exactly the same as the corresponding RRs in the top node of the subzone.
Since name servers are always associated with zone boundaries, NS RRs are only found at nodes which are the top node of some zone.
In the data that makes up a zone, NS RRs are found at the top node of the zone (and are authoritative) and at cuts around the bottom of the zone (where they are not authoritative), but never in between.
1.2. RFC 2181
6. Zone Cuts
- The DNS tree is divided into "zones", which are collections of domains that are treated as a unit for certain management purposes. Zones are delimited by "zone cuts". Each zone cut separates a "child" zone (below the cut) from a "parent" zone (above the cut). The domain name that appears at the top of a zone (just below the cut that separates the zone from its parent) is called the zone's "origin". The name of the zone is the same as the name of the domain at the zone's origin. Each zone comprises that subset of the DNS tree that is at or below the zone's origin, and that is above the cuts that separate the zone from its children (if any). The existence of a zone cut is indicated in the parent zone by the existence of NS records specifying the origin of the child zone. A child zone does not contain any explicit reference to its parent.
zone cutとNSレコードの密接な関連が書かれている。-- ToshinoriMaeno 2016-03-22 23:31:41