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Applying HD Ratio to all IPv4 allocations

  • From: Michael.Dillon
  • Date: Mon Feb 09 09:08:17 2004

I'm going to present the following policy change at the ARIN
meeting in April. In a few days I have to submit the exact wording
to ARIN and I'm looking for comments before I do this.
Basically this policy loosens the rigid 80% utilization 
requirement in a progressive fashion to recognize the
inevitable overhead of hierarchy in larger networks.

1. All requests for additional IPv4 address space shall require the 
   efficient utilization of the sum total of all existing allocations 
   including all space reassigned to customers, if any.

2. The HD(Host Density) ratio of the sum total of all previous allocations 

   shall be greater than or equal to .966 and the HD ratio of the most 
   recent allocation shall be greater than or equal to .930 in order to 
   receive additional space.

3. The HD ratio is calculated as log(utilized IPv4 addresses) divided
   by log(total addresses in all previous allocations). In this formula, 
   log refers to the natural logarithm.


The HD ratio was proposed as a way to determine allocation usage 
for IPv6 address allocations. For more details on this, please refer to 
RFC 3194 <>. There is some detailed 
background discussion about applying the HD ratio to IPv4 allocations in a 

proposal by Paul Wilson posted to the APNIC mailing list on Aug 7, 2003
and he presented the it to the annual APNIC policy meeting using these 
I am not suggesting that ARIN should adopt the APNIC proposal and although
Paul invents a new name for the HD ratio, I prefer to keep the original 

The basic thrust of this proposal is to replace the rigid 80% usage 
by the more flexible HD ratio and to shift the emphasis away from the last 

allocated block to include the total allocated address space. To that end, 

the .930 criterion for the last block is a lot looser than the existing 
requirements for the last block. This is because the utilization threshold 

establishes a time buffer between the beginning of an ARIN application for 

additional addresses and the final deployment of new addresses in the 
operational network. By using a looser criterion as network size grows, we 

are also expanding this time buffer. This recognizes that the economy is 
more dependent than ever on the smooth running of our networks and we 
should not artificially force larger members to operate with virtually 
no safety buffers for implementing new addresses. This safety buffer size 
is important because larger networks have more involved processes for 
changes to their network and these processes take time.

Paul Wilson's paper contains ample discussions of the technical 
for using the HD ratio. I have proposed that we use the .966 number that 
suggests, I believe there may be valid arguments for reducing this 
perhaps to .960.

Michael Dillon
Capacity Planning, Prescot St., London, UK
Mobile: +44 7900 823 672    Internet: [email protected]
Phone: +44 20 7650 9493    Fax: +44 20 7650 9030