number of ports available. If set to -1, the watermark is
disabled. Otherwise when the usage percentage becomes lower
than or equal to natPoolWatermarkLow, a notification is
sent. The NAT may also start behaving in low usage mode
(this is implementation-defined).
The pool's current usage percentage can be computed by
summing (natPoolRangeAllocations -
natPoolRangeDeallocations) over all address ranges
belonging to this pool, then dividing by the total number of
IP addresses in this pool and by the size of the port range
in this pool (natPoolPortMax - natPoolPortMin + 1).