I'm assuming you're not using a router.
I've got a Linksys cable modem, and a Linksys 8-port router/switch. The cable modem does all of the communication w/ my ISP, and then the router distributes that connection to each device on my home LAN. Each device in turn is simply set to DHCP. No special settings required on the client end.
That might make your life easier. They're cheap enough now, and that would save you the hassle of unplugging your Mac when you want to use the PS2 on the internet.