The value of the object tmnxLocUsrDbDhcpIpv6Pfx specifies
the IPv6 prefix of this host.
A value of 0::0 means that the prefix-address is not configured.
The value of tmnxLocUsrDbDhcpIpv6Pfx must be set together with
the value of tmnxLocUsrDbDhcpIpv6PfxLen.