The H-Density Ratio for Address Assignment Efficiency An Update on the H ratio (RFC3194)
Original Publication Date: 2001-Nov-01
Included in the Prior Art Database: 2001-Nov-21
Internet Society Requests For Comment (RFCs)
A. Durand: AUTHOR [+2]
This document provides an update on the "H ratio" defined in RFC 1715. It defines a new ratio which the authors claim is easier to understand.
Network Working Group A. Durand
Request for Comments: 3194 SUN Microsystems
Updates: 1715 C. Huitema
Category: Informational Microsoft
The Host-Density Ratio for Address Assignment Efficiency:
An update on the H ratio
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document provides an update on the "H ratio" defined in RFC
1715. It defines a new ratio which the authors claim is easier to
1. Evaluating the efficiency of address allocation
A naive observer might assume that the number of addressable objects
in an addressing plan is a linear function of the size of the
address. If this were true, a telephone numbering plan based on 10
digits would be able to number 10 billion telephones, and the IPv4 32
bit addresses would be adequate for numbering 4 billion computers
(using the American English definition of a billion, i.e. one
thousand millions.) We all know that this is not correct: the 10
digit plan is stressed today, and it handles only a few hundred
million telephones in North America; the Internet registries have
started to implement increasingly restrictive allocation policies
when there were only a few tens of million computers on the Internet.
Addressing plans are typically organized as a hierarchy: in
telephony, the first digits will designate a region, the next digits
will designate an exchange, and the last digits will designate a
subscriber within this exchange; in computer networks, the most
significant bits will designate an address range allocated to a
network provider, the next bits will designate the network of an
organization served by that provider, and then the subnet to which
the individual computers are connected. At each level of the
Durand & Huitema Informational [Page 1]
RFC 3194 An update on the H ratio November 2001
hierarchy, one has to provide some margins: one has to allocate more
digits to the region code than the current number of regions would
necessitate, and more bits in a subnet than strictly required by the
number of computers. The number of elements in any given level of
the hierarchy will change over time, due to growth and mobility.
If the current allocation is exceeded, one has to engage in
renumbering, which is painful and expensive. In short, trying to
squeeze too many objects into a hierarchical address space increases
the level of pain endured by operators and subscribers.
Back in 1993, when we were debating the revision of the Internet
Protocol, we wondered what the acceptable ratio of utilization was of
a given addressing plan. Coming out with such a ratio was useful to
assess how many computers could be connected to the Internet with the
current 32-bit addresses, as well as to decide the size of the next
generation addresses. The second point is now deci...