RFC 2101 (rfc2101) - Page 2 of 13
IPv4 Address Behaviour Today
Alternative Format: Original Text Document
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