If you have more than one Airport on your network and your Mac is having intermittent difficulty connecting to the Internet, it may be that your computer is having trouble roaming between the multiple Airports.
By default, your Mac is not configured to roam very quickly. Instead, the computer will attempt to lock itself on to a router and try to stay connected. The reason you have multiple Airports is to extend coverage around your home. So instead of locking in, you want to configure your Mac to connect to the strongest signal. If you are locked in to a preferred Airport and that Airport gets too far out of range to hold a strong connection, it will appear as if you are disconnected from the Internet.
Follow these steps to fix:
1. Open the Terminal program (if you are unsure, go to Spotlight and type "Terminal" and hit Return.
2. Copy and paste the following command into your terminal window:
sudo /System/Library/PrivateFrameworks/Apple80211.framework/Versions/A/Resources/airport prefs JoinMode=Strongest JoinModeFallback=DoNothing
3. The system will ask for your administrator password - Enter it when requested
4. Close Terminal
That is all. Your computer will now select the router with the strongest signal. The time it takes to roam between Airports will be greatly reduced.
Other optimization suggestions:
1. insure all Airports on your network are assigned to Automatic channel selection
2. All Airports must be configured with the same Wi-Fi network name