The network mask assigned to this local server pool. This object must
be set in conjunction with usdDhcpLocalServerPoolNetworkIpAddress.
Modification of extant values for this object require that it first be
cleared (via set to null IP mask) and then modified (via set to new IP
mask). Only configurable/valid for entries with the index name
(usdDhcpLocalServerPoolName) 'default'.