its zones.  For resolvers, the configuration data identifies the
      name servers which should be the primary sources of information.

      The domain system defines procedures for accessing the data and
      for referrals to other name servers.  The domain system also
      defines procedures for caching retrieved data and for periodic
      refreshing of data defined by the system administrator.

      The system administrators provide:

         The definition of zone boundaries

         Master files of data

         Updates to master files

         Statements of the refresh policies desired

      The domain system provides:

         Standard formats for resource data

         Standard methods for querying the database

         Standard methods for name servers to refresh local data from
         foreign name servers

DOMAIN NAME SPACE

   Name space specifications and terminology

      The domain name space is a tree structure.  Each node and leaf on
      the tree corresponds to a resource set (which may be empty).  Each
      node and leaf has an associated label.  Labels are NOT guaranteed
      to be unique, with the exception of the root node, which has a
      null label.  The domain name of a node or leaf is the path from
      the root of the tree to the node or leaf.  By convention, the
      labels that compose a domain name are read left to right, from the
      most specific (lowest) to the least specific (highest).

      Internally, programs that manipulate domain names represent them
      as sequences of labels, where each label is a length octet
      followed by an octet string.  Because all domain names end at the
      root, which has a null string for a label, these internal



 

RFC 882                                                    November 1983
                                  Domain Names - Concepts and Facilities


      representations can use a length byte of zero to terminate a
      domain name.  When domain names are printed, labels in a path are
      separated by dots (".").  The root label and its associated dot
      are omitted from printed domain names, but the root can be named
      by a null domain name (" " in this memo).

      To simplify implementations, the total number of octets that
      represent label octets and label lengths is limited to 255.  Thus
      a printed domain name can be up to 254 characters.

      A special label is defined that matches any other label.  This
      label is the asterisk or "*".  An asterisk matches a single label.
      Thus *.ARPA matches FOO.ARPA, but does not match FOO.BAR.ARPA.
      The asterisk is mainly used to create default resource records at
      the boundary between protocol families, and requires prudence in
      its use.

      A domain is identified by a domain name, and consists of that part
      of the domain name space that is at or below the domain name which
      specifies the domain.  A domain is a subdomain of another domain
      if it is contained within that domain.  This relationship can be
      tested by seeing if the subdomain's name has the containing
      domain's name as the right part of its name.  For example, A.B.C.D
      is a subdomain of B.C.D, C.D, D, and " ".

      This tree structure is intended to parallel the administrative
      organization and delegation of authority.  Potentially, each node
      or leaf on the tree can create new subdomains ad infinitum.  In
      practice, this delegation can be limited by the administrator of
      the name servers that manage the domain space and resource data.

      The following figure shows an example of a domain name space.

                                   |                            
                +------------------+------------------+         
                |                  |                  |         
              COLORS            FLAVORS             TRUTH       
                |                  |                            
          +-----+-----+            |                            
          |     |     |         NATURAL                         
         RED  BLUE  GREEN          |                            
                                   |                            
                   +---------------+---------------+            
                   |               |               |            
               CHOCOLATE        VANILLA        STRAWBERRY       

      In this example, the root domain has three immediate subdomains: