The following is a “screenshot simulation” of the Network Settings for OS X v10.5 (Leopard). Your ISP will determine what the correct settings are for you. When in doubt, check with them.
Since its been a while since I posted a screenshot sim (also referred to as screenshot simulators) I’ll give a brief explanation of their intent. This simulation can be used when helping someone configure their network/Internet settings remotely – and you’re not in front of a Mac running Leopard. Anything high-lighted is clickable.
This screenshot sim will launch in a new window/tab when you select System Preferences… from the Apple icon in the menu bar.