.mobi domains (short for "mobile") are the first Top Level Domains (TLD) specifically designed for web pages which have been adapted to mobile devices. Having a .mobi will indicate that you have created a website that has been optimised for viewing and navigating on a smaller mobile device.
Start pages for .mobi domains must result in a page using the XHTML Mobile Profile doctype. If the .mobi website redirects to a home page, then all intermediate pages that are supplied in the course of redirection must also comply with this rule.
Websites utilising a subdomain such as www.domain.com must also provide a website using domain.com (Both www.domain.com and domain.com will already reference the same site by default when transferring or registering a new .mobi domain to 1&1).
HTML Frames are not permitted for websites available through a .mobi domains name. For this reason, the 1&1 Control Panel does not allow to set a Frame Redirect for .mobi domains.