If your device has network adaptor interface at client PC, it should be either in NDIS mode or ICS mode? Using which mode depends on what your device operating methodology. Usually, dongle type of data card is likely to be NDIS mode. In this mode, after the device accesses the network and enables data connection, the network will assign an IP address to the device. Then, this IP address will also be the IP address for this network adaptor interface at client PC. Thus, client PC will have the IP address same as what network assigned.
For smartphone, e.g. Window Mobile or Android, with USB connection, it's usually ICS mode. In this mode, after the data connection is setup, the network will similarly assign IP address to the device. However, the device will use it within the device and setup another internal LAN with the connection to the client PC. The device acts as a internet router and setup NAT. The network adaptor interface at client PC will have an internal IP address decided by the device, e.g. 192.168.42.100. It's implementation dependent