How2 redirect visitors to a web site’s mobile subdomain?

We have a business web site and want to create a mobile subdomain for it. What’s the best way to identify mobile visitors arriving at our main URL and redirect them to our mobile subdomain, transparently?

Our web host, 1and1, can detect mobile visitors they say, but neither they nor I know how to access that ‘flag’ (or whatever) and use it to trigger the redirection.

Help, advice, or a point in the right direction appreciated. We’re mobile newbies. Thanks. — Jim