Open Network Preferences, and:
Your IP address is either static or dynamic. Static: you put in your IP info. Dynamic: your gateway provides the correct IP info.
Open Terminal.app, and type:
That will show you what network interfaces you have. One of these is your primary route and your default gateway. You can only have one default gateway. Even though you may have many interfaces (ethernet, wireless and firewire, etc), there's only one default way to go if the request is not local.
So check for default route by typing in Terminal.app:
netstat -nr | more
The top two resulting lines will tell you who's the boss.
Then ping the gateway.
ping <IP address>
If it doesn't work, reboot and try again. You should at a minimum ping your gateway and ping your DNS before it's worth it to troubleshoot individual applications.