Level one connectivity is through a gateway that allows two networks to communicate but users of the non Internet network (eg Compuserve) are limited in their ability to use all the tools of the Internet.
Level Two connectivity is through a dial up terminal connection using a modem. Your computer acts like a terminal on a mainframe host for example. Therefore if you decide to download a file from the Internet it is stored on the host computer and not your personal computer. You then have to download from the host to your personal computer. Level Three connectivity is the highest and most expensive form of connectivity - this is what universities have - they are connected to the Internet 24 hours a day, seven days a week.
Finally there is SLIP or Serial Line Internet Protocol and PPP or Point to Point Protocol. You connect to the Internet via a high speed modem whenever you want and your computer becomes a node on the Internet. You need only pay for when you are connected and you are able to download directly onto your personal computer.
