The H Ratio for Address Assignment Efficiency (RFC1715)
Original Publication Date: 1994-Nov-01
Included in the Prior Art Database: 2019-Feb-12
Internet Society Requests For Comment (RFCs)
This document was submitted to the IETF IPng area in response to RFC 1550. This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind.
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 firstname.lastname@example.org mailing list.
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...