The interface is the actual IP address given to your network card by the DHCP server of your router (or the static IP you gave your machine).
Terminology is different for every device but normally you have the path to take for every interface. On a Windows computer, type route print in CMD to see the routing table of your computer (which can act just like a router if you enable routing in its settings). A packet only contains the final destination IP address otherwise it would be lost.
