The right domain name for your main website is a big deal. Businesses spend time, money & energy thinking about the length, the keywords, and other factors they think will help their site rank well in the search engines. Many choose to pay serious money to buy a dot come rather than settle for some other Top Level Domain (TLD).
Selecting a domain name for your mobile friendly website is much less difficult. In fact, you may choose not to use another domain name at all. Visitors are unlikely to even be aware your mobile friendly website is on a different domain. They are not going to be typing it into their browser window. Visitors will go to your main website which will automatically redirect them to the mobile site. This will be invisible to most. (The geeks might notice the domain change in their browser address box).
Below are some choices you have with some pros & cons for each.
Use the same domain but put the mobile friendly content into its' own folder. For example www.MyMainSite.com/mobile. This works if the webmaster responsible for your main site is also doing your mobile site. It's probably not such a great idea if the responsible people are different. There is lots of opportunity for one party to make changes that mess up the other. Then you get to referee the geek fight over who was responsible.
Create a subdomain. This is usually done by creating m.MyMainSite.com and Google has indicated they understand this, will not penalize it for duplicate content, etc. Using "m" is just a convention; it could be anything. With this arrangement the two sites can have different logins which prevents the conflicts mentioned above. Your existing internet person or someone at your hosting company has to set this up because it requires access to the same name server (DNS) as your main site.
Register another domain. There really is a TLD called mobi as in mobile. It has not been especially popular, although that may be changing, so MyMainSite.mobi is probably still available. I like this approach because I can register the domain & publish a draft mobile friendly site there without any active support from your internet support staff and/or access to your existing site. Google has said they recognize this as another reasonable way to set things up.
This approach may cost a little more because you have an extra domain registration fee and another hosting account, but the charges will be minor. If I build your website these charges are include in my fee.
As someone who specializes in building mobile friendly sites for customers this works really well. I already have hosting accounts with providers I know and trust, with control panels I have worked with for years. When something goes wrong or changes are needed I can take care of things very efficiently. I can't be accused of taking down the main site by doing something dumb. The reverse is also true. I don't have to worry about the main website support people "forgetting" I am there and taking me down. I don't have to tip toe around unfamiliar systems to do my work.
The benefits are so compelling this is the only technique I use.
