In NAT mode, the guest network interface is assigned to the IPv4 range 10.0.x.0/24 by default where x corresponds to the instance of the NAT interface +2 of that VM. So x is 2 if there is only once NAT instance active. In that case the guest is assigned to the address 10.0.2.15, the gateway is set to 10.0.2.2 and the name server can be found at 10.0.2.3.

If, for any reason, the NAT network needs to be changed, this can be achieved with the following command:

VBoxManage modifyvm "My VM" -natnet1 "192.168/16"

This command would reserve the network addresses 192.168.0.0 ... 192.168.254.254 for the first NAT network instance of "My VM". The guest IP would be assigned to 192.168.0.15 and the default gateway could be found at 192.168.0.2.