The most likely reason you get the same IP address is that the DHCP server keeps track of which MAC has which IP address. VB uses two methods to get on the network. The first it tries to use the Guest MAC address, and if that fails, it will use the Host NIC MAC address. The latter is used for you now, so the DHCP gives the same IP address. Setting it to static like Vbox4me2 suggested will surely help.