The length of the IP network mask used to refine the range of
IP addresses defined by etsysNatPoolFirstIpAddr and
etsysNatPoolLastIpAddr in defining the range of IP addresses
for this NAT Pool entry. The values for the index objects
etsysNatPoolFirstIpAddr, etsysNatPoolLastIpAddr and
etsysNatPoolPrefixLen must be consistent. If a prefix length
of '0' is specified than all addresses in the range are used.