c. A network application fails or refuses to connect! What can I do?

The first thing to check is the firewall. Windows have an inbuilt firewall since Windows XP SP3. It is important to set firewall to allow your application to communicate in the intranet domain (your local domain). Setting the firewall right requires defining inbound and outbound rules. In Windows 7 go to Start à Command Panel à Administrative tools and click on Windows Firewall with Advanced Security. Then allow your program (EXE file) to communicate through the firewall on any port. From the upper left selection select Inbound rules view. Click in New Rule (upper right corner text) and define an appropriate rule.

You may also opt for allowing all local domain communication by choosing Custom, click Next button, All programs, click Next button, Any (protocol), click Next button, twice Any IP address, click Next button, Allow communication, click Next button and enter rule name and then confirm. Repeat the same for the Outbound rules.

If you have Windows XP, you can also find Firewall icon amongst Administrative tools on Command Panel.

You may also opt for turning the firewall down, if there is a common firewall for your intranet (local domain).