EarthLink: How do PCs establish Internet connection? DHCP?

Hello all,

A friend of mine has recently had a cable modem installed for her internet connection with EarthLink. When she plugs the ethernet cable of her laptop into the modem, she gets Internet access. So far so good. The next step is to buy and install a router to construct a home network, and here she's stuck.

I also have a cable modem at home and my provider is Roadrunner. They give an IP address via DHCP to whatever connected to the modem. So, I bought a router and plugged it to the modem, and then Roadrunner gives my router a global IP address; I plug my PCs to the switch of the router and then the router gives private addresses (192.168.xxx.xxx) to my PCs via DHCP. Nothing is easier than that!

But, I don't understand what EarthLink does. I checked the EarthLink website and found this kind of softwarre is used

Could somebody tell me what EarthLink does to give internet access? Can she install a router for herself?

Thank you, Ryo