It depends on how you connect, and what cable/DSL modem you have. Usually, you can just run cat3/5 cable between your computer and the modem. If you're not planning on moving the laptop much, and it's close to the modem, you may also be able to use a USB connection, though in some instances you may need a proprietary,ISP supplied driver for this to work (BellSouth was notorious for this)
Also, if you have been running the modem with a router, you may need to take the modem out of bridged mode in order for it to work properly with a single computer. If this is the case, there should be a reset button somewhere on the back of the modem that will restore the modem to default factory/ISP settings. All Login info stored in the modem will have to be re-entered afterwards, though.
That being said, the built in LAN adapter in laptops will sometimes die, in which case, you'll have to get a PCMCIA, or USB LAN adapter in order to connect.
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