How to Troubleshoot Application Connectivity Problems

Sometimes, you might be able to access the network with some applications but not others. For example, you might be able to download your e-mail but not access Web servers. Or, you might be able to view pages on a remote Web server but not connect to the computer with Remote Desktop.

Several issues might cause these symptoms (in rough order of likelihood):

• The remote service is not running. For example, Remote Desktop might not be enabled on the remote computer.
• The remote server has a firewall configured that is blocking that application's communications from your client computer.
• A firewall between the client and server computer is blocking that application's communications.
• Windows Firewall on the local computer might be configured to block the application's traffic.
• The remote service has been configured to use a non-default port number. For example, Web servers typically use TCP port 80, but some administrators might configure TCP port 81 or a different port.

To troubleshoot an application connectivity problem, follow these steps:

1. Before you begin troubleshooting application connectivity, first verify that you do not have a name resolution problem. To do this, open a command prompt and run the command Nslookup servername. If Nslookup does not display an answer similar to the following example, you have a name resolution problem. See the section titled "How to Troubleshoot Name Resolution Problems" later in this tutorial.
   C:\>nslookup sourcedaddy.com

   Non-authoritative answer:
   Name: sourcedaddy.com
   Addresses: 50.22.4.99
   

2. Identify the port number used by the application.

Table next section lists port numbers for common applications.