RFC 3194 (rfc3194) - Page 2 of 7
The H-Density Ratio for Address Assignment Efficiency An Update on the H ratio
Alternative Format: Original Text Document
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 decided, with 128-bits
addresses for IPv6, but the first question is still relevant:
knowing the capacity of the current address plan will help us predict
the date at which this capacity will be exceeded.
Participants in the IPNG debates initially measured the efficiency of
address allocation by simply dividing the number of allocated
addresses by the size of the address space. This is a simple
measure, but it is largely dependent on the size of the address
space. Loss of efficiency at each level of a hierarchical plan has a
multiplicative effect; for example, 50% efficiency at each stage of a
three level hierarchy results in a overall efficiency of 12.5%. If
we want a "pain level indicator", we have to use a ratio that takes
into account these multiplicative effects.
The "H-Ratio" defined in RFC 1715 proposed to measure the efficiency
of address allocation as the ratio of the base 10 logarithm of the
number of allocated addresses to the size of the address in bits.
This provides an address size independent ratio, but the definition
of the H ratio results in values in the range of 0.0 to 0.30103, with
typical values ranging from 0.20 to 0.28. Experience has shown that
these numbers are difficult to explain to others; it would be easier
to say that "your address bits are used to 83% of their H-Density",
and then explain what the H-Density is, than to say "you are hitting
a H ratio of 0.25" and then explain what exactly the range is.
This memo introduces the Host Density ratio or "HD-Ratio", a proposed
replacement for the H-Ratio defined in RFC 1715. The HD values range
from 0 to 1, and are generally expressed as percentage points; the
authors believe that this new formulation is easier to understand and
more expressive than the H-Ratio.
Durand & Huitema Informational