An enumerated value which specifies the address type
of the target.
This object must be used along with the
crttMonIPEchoAdminTargetAddress
object as it identifies the address family of the
address specified by that object. If the value
of crttMonIPEchoAdminTargetAddress is a zero-length
string (e.g., because an IPv4 address is specified by
rttMonEchoAdminTargetAddress), this object contains
the value 'unknown'.