The article below gives some things you can try.
From apple at
http://docs.info.apple.com/article.html?artnum=106798
Solution
Tip: Users of Mac OS X 10.4 have the option of clicking the "Assist me" button in the Network preference pane, then following the onscreen prompts for network diagnostics. However, the following information is good for any version of Mac OS X.
This may be caused by an actual loss of your Internet connection, or it may be caused by having incorrect domain name system (DNS) settings. Here is a simple test. At the affected computer, type this in the address field of your Web browser:
http://17.254.0.91
If that address takes you to
www.apple.com, then you most likely have a DNS issue and need to continue reading this document.
If this address does not take you to
www.apple.com, then you need to skip to the "Troubleshooting Internet connections" section below.
Note: When you access the
www.apple.com front page in this manner, you may only get a partial page load, usually text with no pictures. A partial page load indicates that you have a connection, and that you should continue troubleshooting DNS before troubleshooting other aspects of your connection.
What does DNS service do?
A DNS server converts DNS names, like "www.apple.com" into their IP address equivalents, such as "17.254.0.91". When you do not have DNS service, your computer cannot find the IP address of the target server based on its DNS name. If you encounter some applications that continue to work, it may be because they are set up to go directly to an IP address, bypassing the need for DNS service. The issue can be resolved by adding a DNS address that is missing, or correcting one that is invalid or mistyped.
For Mac OS X
This is also associated with slow system startup. For more information on that symptom and specific steps to resolve this issue in Mac OS X , see technical document 106797, "Mac OS X: Slow Startup, Pauses at 'Initializing network' or 'Starting network time'".
For Mac OS 9
Contact your network administrator or Internet service provider to get a valid DNS server address. Only they can provide this information to you. You may use more than one DNS server, which ensures your service in the event that one becomes unavailable. After you have the information, follow these steps:
1. Open the TCP/IP control panel.
2. Locate the "Name Server addr." field. If you see "will be supplied by server" next to this field, then continue to the next step. If you do not see "will be supplied by server", then skip to Step 6.
3. Choose User Mode from the Edit menu.
4. Select the radio button for Advanced.
5. Click OK.
6. Type your DNS address in the "Name Server addr." field. If you have more than one, press Return at the end of each address to place the next on a new line.
7. Close the control panel, clicking Save when prompted.
Note: Some networks and Internet service providers use a protocol that allows your computer to discover DNS service automatically on the network without manually typing the address. This is why you may not be provided with a DNS server address for your network. If your network provides automatic discovery, you should only encounter the symptoms if you have specified an incorrect DNS address, versus leaving the address field empty, which would not affect your computer. Even when automatic discovery is provided, it is still a valid step to manually enter DNS addresses when troubleshooting an Internet connection issue.
Troubleshooting Internet connections
These technical documents provide troubleshooting information for all aspects of your Internet connection. See:
106748: "Mac OS X: Troubleshooting a Dial-Up/PPP Internet Connection"
106764: "Mac OS X: Opening Web Browser Before Connecting to Internet Results in Alert Message"
106749: "Mac OS X: Troubleshooting a Cable Modem or DSL Internet Connection"
106747: "Mac OS X: Troubleshooting a PPPoE Internet Connection"
