TCP/IP Tutorial and Technical Overview - IBM Redbooks

names within the zone are administered independently of names in other zones.
Authority over zones is vested in the name servers.
Normally, the name servers that have authority for a zone will have domain
names belonging to that zone, but this is not required. Where a domain contains
a subtree that falls in a different zone, the name server or servers with authority
over the superior domain are said to delegate authority to the name server or
servers with authority over the subdomain. Name servers can also delegate
authority to themselves; in this case, the domain name space is still divided into
zones moving down the domain name tree, but authority for two zones is held by
the same server. The division of the domain name space into zones is
accomplished using resource records stored in the Domain Name System.
At the top-level root domain there is an exception to this. There is no higher
system to which authority can be delegated, but it is not desirable to have all
queries for fully qualified domain names to be directed to just one system.
Therefore, authority for the top-level zones is shared among a set of root name
servers 1 coordinated by the ICANN.
To better illustrate the process of resolving a symbolic name to an IP address,
consider a query for myHost.myDept.myCorp.com, and let us assume that our
name server does not have the answer already in its cache. The query goes to
the .com root name server, which in turn forwards the query to a server with an
NS record for myCorp.com. At this stage, it is likely that a name server has been
reached that has cached the needed answer. However, the query could be
further delegated to a name server for myDept.myCorp.com
As a result of this scheme:
► Rather than having a central server for the database, the work that is involved
in maintaining this database is off-loaded to hosts throughout the name
space.
► Authority for creating and changing symbolic host names and responsibility
for maintaining a database for them is delegated to the organization owning
the zone (within the name space) containing those host names.
► From the user's standpoint, there is a single database that deals with these
address resolutions. The user might be aware that the database is
distributed, but generally need not be concerned about this.
1 At the time of writing, there were 13 root servers.
Chapter 12. Directory and naming protocols 431