The Domain Naming Convention for Internet User Applications
RFC 819

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Network Working Group                                  Zaw-Sing Su (SRI)
Request for Comments: 819                               Jon Postel (ISI)
                                                             August 1982

      The Domain Naming Convention for Internet User Applications

1.  Introduction

   For many years, the naming convention "<user>@<host>" has served the
   ARPANET user community for its mail system, and the substring
   "<host>" has been used for other applications such as file transfer
   (FTP) and terminal access (Telnet).  With the advent of network
   interconnection, this naming convention needs to be generalized to
   accommodate internetworking.  A decision has recently been reached to
   replace the simple name field, "<host>", by a composite name field,
   "<domain>" [2].  This note is an attempt to clarify this generalized
   naming convention, the Internet Naming Convention, and to explore the
   implications of its adoption for Internet name service and user
   applications.

   The following example illustrates the changes in naming convention:

      ARPANET Convention:   Fred@ISIF
      Internet Convention:  Fred@F.ISI.ARPA

   The intent is that the Internet names be used to form a
   tree-structured administrative dependent, rather than a strictly
   topology dependent, hierarchy.  The left-to-right string of name
   components proceeds from the most specific to the most general, that
   is, the root of the tree, the administrative universe, is on the
   right.

   The name service for realizing the Internet naming convention is
   assumed to be application independent.  It is not a part of any
   particular application, but rather an independent name service serves
   different user applications.

2.  The Structural Model

   The Internet naming convention is based on the domain concept.  The
   name of a domain consists of a concatenation of one or more <simple
   names>.  A domain can be considered as a region of jurisdiction for
   name assignment and of responsibility for name-to-address
   translation.  The set of domains forms a hierarchy.

   Using a graph theory representation, this hierarchy may be modeled as
   a directed graph.  A directed graph consists of a set of nodes and a

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RFC 819                                                     August 1982;

   collection of arcs, where arcs are identified by ordered pairs of
   distinct nodes [1].  Each node of the graph represents a domain.  An
   ordered pair (B, A), an arc from B to A, indicates that B is a
   subdomain of domain A, and B is a simple name unique within A.  We
   will refer to B as a child of A, and A a parent of B.  The directed
   graph that best describes the naming hierarchy is called an
   "in-tree", which is a rooted tree with all arcs directed towards the
   root (Figure 1). The root of the tree represents the naming universe,
   ancestor of all domains.  Endpoints (or leaves) of the tree are the
   lowest-level domains.

                         U
                       / | \
                     /   |   \          U -- Naming Universe
                    ^    ^    ^         I -- Intermediate Domain
                    |    |    |         E -- Endpoint Domain
                    I    E    I
                  /   \       |
                 ^     ^      ^
                 |     |      |
                 E     E      I
                            / | \
                           ^  ^  ^
                           |  |  |
                           E  E  E

                                Figure 1
                 The In-Tree Model for Domain Hierarchy

   The simple name of a child in this model is necessarily unique within
   its parent domain.  Since the simple name of the child's parent is
   unique within the child's grandparent domain, the child can be
   uniquely named in its grandparent domain by the concatenation of its
   simple name followed by its parent's simple name.

      For example, if the simple name of a child is "C1" then no other
      child of the same parent may be named "C1".  Further, if the
      parent of this child is named "P1", then "P1" is a unique simple
      name in the child's grandparent domain.  Thus, the concatenation
      C1.P1 is unique in C1's grandparent domain.

   Similarly, each element of the hierarchy is uniquely named in the
   universe by its complete name, the concatenation of its simple name
   and those for the domains along the trail leading to the naming
   universe.

   The hierarchical structure of the Internet naming convention supports
   decentralization of naming authority and distribution of name service
   capability.  We assume a naming authority and a name server

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RFC 819                                                     August 1982;

   associated with each domain.  In Sections 5 and 6 respectively the