Think of it this way: ISP<-->compatible modem (purchased or leased from the ISP, connected to the wires coming from the outside world into your house)<-->WAN connection to a router<-->LAN connection between the router and your computer (the router is not necessary unless you will be connecting more than one computer to the modem. If no router, the WAN connection goes right to the computer -- some modems have built in routers, which can be determined by the modem having several LAN ports. The double arrowheads stand for addresses, which are variations of the form of "192.168.1.10", etc., used between the nodes. These can be either static (assigned manually) or "DHCP", or "dynamic host configuration protocol", in which the two pieces of apparatus on either side of an arrow negotiate with each other to establish the address numbers they will be using for communicating. Each address, i.e. each left and right pointing arrowhead, must be unique, even when established using DHCP. "Modem" stands for "modulate and demodulate".