Google’s Matt Cutts answers the question “Is there a way to tell Google there is a mobile version of a page, so it can show the alternate page in mobile search results? Or similarly, that a page is responsive and the same URL works on mobile?”
Cutts qualifies his response and states that he is referring to smartphones when talking about mobile versions. Obviously, webmasters want users to go to the mobile or desktop versions of their sites as required.
According to Cutts, there are a couple of ways of achieving this. One is to use a responsive design which serves up the same site to mobile or PC users. The second way is to have two separate versions of the site which points users to either one.
As the video explains, if taking the second option, then it’s important to have a rel=”alternate” tag on the desktop which points to the mobile. This allows the Googlebots to understand that the two sites are connected. Conversely, he also advises to include a rel=”canonical” on the mobile site.