To connect a Windows server to an existing DC (Domain Controller) server, you need to follow these steps:

1. First, make sure that the Windows server is configured with a static IP address and that it has access to the network where the DC server is located.
2. On the Windows server, open the Control Panel and click on “System and Security”.
3. Click on “System” and then click on “Advanced system settings”.
4. In the System Properties window, click on the “Computer Name” tab and then click on the “Change” button.
5. In the Computer Name/Domain Changes window, select the “Domain” option and enter the name of the domain where the DC server is located.
6. Click on “OK” and enter the credentials of a domain administrator account when prompted.
7. Once the computer has been joined to the domain, you will need to restart the server for the changes to take effect.
8. After the server has restarted, you should be able to log in to the server using domain credentials and access resources on the DC server.

That’s it! Your Windows server is now connected to the existing DC server.