If you have only one package, you will land on the Administration page. If you have more than one package, select the package in question to reach its Administration page.
Package Overview
Step 2
Click the IP Address Overview link from the 1&1 Servers panel.
Choose IP Address Overview
Step 3
Check the box next to an IP address and click the Reverse Mapping button.
Click the Reverse Mapping button
Step 4
Enter the domain name and click the OK button.
Please note:
Please note that all DNS changes (such as a CNAME), even though effective on 1&1 server within minutes, may take up to 1 hour until are recognized by all servers/computers on the Internet.
Enter the domain and click OK
For additional information, you may want to reference: