hi everybody i know this may be really stupid question but i really need this information

my friend has ntl cable broadband working at her house and i wanted to use it too and just connect the ethernet cable to my laptop, the problem is when i try to install and configure ntl software when it says to plug in the cable i do it and then i get an error that it cant renew my ip or something like that :/:/ what should i do to be able to freely switch cable between our two computers? (cause i dont need bb all the time so i wasnt thinking bout router yet) please help