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The H-Density Ratio for Address Assignment Efficiency An Update on the H ratio (RFC3194) Disclosure Number: IPCOM000005941D
Original Publication Date: 2001-Nov-01
Included in the Prior Art Database: 2019-Feb-14
Document File: 7 page(s) / 10K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

A. Durand: AUTHOR [+1]

Related Documents

10.17487/RFC3194: DOI


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. This memo provides information for the Internet community.

This text was extracted from a PDF file.
This is the abbreviated version, containing approximately 24% of the total text.

Network Working Group A. Durand Request for Comments: 3194 SUN Microsystems Updates: 1715 C. Huitema Category: Informational Microsoft November 2001

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 Notice

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 understand.

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...