RFC 2101 (rfc2101) - Page 2 of 13

IPv4 Address Behaviour Today

RFC 2101              IPv4 Address Behavior Today          February 1997


   This clarification is intended to assist protocol designers, product
   implementors, Internet service providers, and user sites. It aims to
   avoid misunderstandings about IP addresses that can result from the
   substantial changes that have taken place in the last few years, as a
   result of the Internet's exponential growth.

   A short section on IPv6 addresses mentions the main points of
   similarity with, and difference from, IPv4.

2. Terminology

   It is well understood that in computer networks, the concepts of
   directories, names, network addresses, and routes are separate and
   must be analysed separately [RFC 1498].  However, it is also
   necessary to sub-divide the concept of "network address" (abbreviated
   to "address" from here on) into at least two notions, namely
   "identifier" and "locator". This was perhaps less well understood
   when RFC 791 was written.

   In this document, the term "host" refers to any system originating
   and/or terminating IPv4 packets, and "router" refers to any system
   forwarding IPv4 packets from one host or router to another.

   For the purposes of this document, an "identifier" is a bit string
   which is used throughout the lifetime of a communication session
   between two hosts, to identify one of the hosts as far as the other
   is concerned. Such an identifier is used to verify the source of
   incoming packets as being truly the other end of the communication
   concerned, e.g. in the TCP pseudo-header [RFC 793] or in an IP
   Security association [RFC 1825].  Traditionally, the source IPv4
   address in every packet is used for this.

   Note that other definitions of "identifier" are sometimes used; this
   document does not claim to discuss the general issue of the semantics
   of end-point identifiers.

   For the purposes of this document, a "locator" is a bit string which
   is used to identify where a particular packet must be delivered, i.e.
   it serves to locate the place in the Internet topology where the
   destination host is attached. Traditionally, the destination IPv4
   address in every packet is used for this. IP routing protocols
   interpret IPv4 addresses as locators and construct routing tables
   based on which routers (which have their own locators) claim to know
   a route towards the locators of particular hosts.

   Both identifiers and locators have requirements of uniqueness, but
   these requirements are different. Identifiers must be unique with
   respect to each set of inter-communicating hosts. Locators must be



Carpenter, et. al.           Informational