[ppml] HD Ratio changes

I've made some changes to the wording of the policy. First of all
I've made the use of the HD Ratio optional. Secondly, I've decided
to use the natural logarithm in the calculation because PERL has
this and not the base 10 logarithm. The end result of the ratio
calculation is identical, but now people will see that is clearly
is easy to add the calculation to PERL IP address management
systems. The Excel function LN(x) is the same as PERL's log(x).
And lastly, I've pulled out the mention of reassignments since
any definition of utilization belongs elsewhere and this was
getting too close.

1. Anyone who has already been allocated 4096 IPv4 addresses or
   more may choose to have additional requests for IPv4 addresses
   evaluated using an HD (Host Density) Ratio calculation to determine 
   sufficient utilization instead of a fixed percentage threshold.

2. All requests for additional IPv4 address space subject to the HD 
   Ratio shall require the efficient utilization of the sum total of 
   all existing allocations. The HD Ratio on the sum total of all 
   existing allocations must be greater than or equal to .966.

3. In addition, the HD ratio of the most recent allocation must be 
   greater than or equal to .930.

4. 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.

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Michael Dillon
Capacity Planning, Prescot St., London, UK
Mobile: +44 7900 823 672    Internet: michael.dillon at radianz.com
Phone: +44 20 7650 9493    Fax: +44 20 7650 9030