Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


Network Working Group C. Huitema Request for Comments: 1715 INRIA Category: Informational November 1994

           The H Ratio for Address Assignment Efficiency

Status of this Memo

 This memo provides information for the Internet community.  This memo
 does not specify an Internet standard of any kind.  Distribution of
 this memo is unlimited.


 This document was submitted to the IETF IPng area in response to RFC
 1550.  Publication of this document does not imply acceptance by the
 IPng area of any ideas expressed within.  Comments should be
 submitted to the author and/or the mailing

Table of Contents

 1.   Efficiency of address assignment . . . . . . . . . . . . . . 1
 2.   Estimating reasonable values for the ratio H . . . . . . . . 2
 3.   Evaluating proposed address plans. . . . . . . . . . . . . . 3
 4.   Security Considerations . . . . . . . . . . . . . . . . . .  4
 5.   Author's Address . . . . . . . . . . . . . . . . . . . . . . 4

1. Efficiency of address assignment

 A substantial part of the "IPng" debate was devoted to the choice of
 an address size. A recurring concept was that of "assignment
 efficiency", which most people involved in the discussion expressed
 as a the ratio of the effective number of systems in the network over
 the theoretical maximum. For example, the 32 bits IP addressing plan
 could in theory number over 7 billions of systems; as of today, we
 have about 3.5 millions of addresses reported in the DNS, which would
 translate in an efficiency of 0.05%.

Huitema [Page 1] RFC 1715 H Ratio November 1994

 But this classic evaluation is misleading, as it does not take into
 account the number of hierarchical elements. IP addresses, for
 example, have at least three degrees of hierarchy: network, subnet
 and host.  In order to remove these dependencies, I propose to use a
 logarithmic scale for the efficiency ratio:
                           log (number of objects)
                       H = -----------------------
                                available bits
 The ratio H is not too dependent of the number of hierarchical
 levels. Suppose for example that we have the choice between two
 levels, encoded on 8 bits each, and one single level, encoded in 16
 bits. We will obtain the same efficiency if we allocate in average
 100 elements at each 8 bits level, or simply 10000 elements in the
 single 16 bits level.
 Note that I use base 10 logs in what follows, because they are easier
 to compute mentally. When it comes to large numbers, people tend to
 use "powers of 10", as in "IPng should be capable of numbering 1 E+15
 systems". It follows from this choice of units that H varies between
 0 and a theoretical maximum of 0.30103 (log base 10 of 2).

2. Estimating reasonable values for the ratio H:

 Indeed, we don't expect to achieve a ratio of 0.3 in practice, and
 the interesting question is to assert the values which can be
 reasonably expected. We can try to evaluate them from existing
 numbering plans. What is especially interesting is to consider the
 moment where the plans broke, i.e. when people were forced to add
 digits to phone number, or to add bits to computer addresses. I have
 a number of such figures handy, e.g.:
  • Adding one digit to all French telephone numbers, moving from 8

digits to 9, when the number of phones reached a threshold of 1.0

   E+7. The log value is 7, the number of bits was about 27 (1 decimal
   digit is about 3.3 bits). The ratio is thus 0.26
  • Expending the number of areas in the US telephone system, making it

effectively 10 digits long, for about 1.0 E+8 subscribers. The log

   value is 8, the number of bits is 33, the ratio is about 0.24
  • Expending the size of the Internet addresses, from 32 bits to

something else. There are currently about 3 million hosts on the

   net, for 32 bits. The log of 3.E6 is about 6.5; this gives a ratio
   of 0.20. Indeed, we believe that 32 bits will still be enough for
   some years, e.g. to multiply the number of hosts by 10, in which
   case the ratio would climb to 0.23

Huitema [Page 2] RFC 1715 H Ratio November 1994

  • Expending the size of the SITA 7 characters address. According to

their documentation, they have about 64000 addressed points in

   their network, scattered in 1200 cities, 180 countries. An upper
   case character provides about 5 bits of addressing, which results
   in an efficiency of 0.14. This is an extreme case, as SITA uses
   fixed length tokens in its hierarchy.
  • The globally-connected physics/space science DECnet (Phase IV)

stopped growing at about 15K nodes (i.e. new nodes were hidden)

   which in a 16 bit space gives a ratio of 0.26
  • There are about 200 million IEEE 802 nodes in a 46 bit space, which

gives a ratio of 0.18. That number space, however, is not

 From these examples, we can assert that the efficiency ratio usually
 lies between 0.14 and 0.26.

3. Evaluating proposed address plans

 Using a reverse computation, we get the following population counts
 in the network:
                  Pessimistic (0.14)     Optimistic (0.26)
    32 bits             3 E+4 (!)           2 E+8
    64 bits             9 E+8               4 E+16
    80 bits           1.6 E+11            2.6 E+27
   128 bits             8 E+17              2 E+33
 I guess that the figure explains well why some feel that 64 bits is
 "not enough" while other feel it is "sufficient by a large margin":
 depending of the assignment efficiency, we are either well below the
 target or well above. But there is no question, in my view, that 128
 bits is "more than enough". Even if we presume the lowest efficiency,
 we are still way above the hyperbolic estimate of 1.E+15 Internet
 It is also interesting to note that if we devote 80 bits to the
 "network" and use 48 bits for "server less autoconfiguration", we can
 number more that E.11 networks in the pessimistic case - it would
 only take an efficiency of 0.15 to reach the E+12 networks hyperbole.
 I guess this explains well why I feel that 128 bits is entirely safe
 for the next 30 year. The level of constraints that we will have to
 incorporate in the address assignment appears very much in line with
 what we know how to do, today.

Huitema [Page 3] RFC 1715 H Ratio November 1994

4. Security Considerations

 Security issues are not discussed in this memo.

5. Author's Address

 Christian Huitema
 INRIA, Sophia-Antipolis
 2004 Route des Lucioles
 BP 109
 F-06561 Valbonne Cedex
 Phone: +33 93 65 77 15
 EMail: Christian.Huitema@MIRSA.INRIA.FR

Huitema [Page 4]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1715.txt · Last modified: 1994/11/02 21:30 by

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki