Even though this should be a default development technique for a mobile site setup for most developers (and SEO professionals) nowadays, there may still be some people who are unaware of this.
An SEO professional wanted to know: is there a difference between using example.com and m.example.com when it comes to Google indexing sites on the mobile version that’s on a subdomain?
John explained that there usually aren’t any issues using m-dot domains in general. This is one of the supported formats that Google can still crawl.
Even so, John doesn’t recommend using an m-dot subdomain at all, and instead recommends the more modern approach of using a responsive design.
He also mentioned that although Google is unable to see what’s on your mobile site on an m-dot, this could very well be due to issues with Google being able to access something rather than the use of the m-dot subdomain setup.
This happens at approximately the 30:07 mark in the video.
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John Mueller Hangout Transcript
Does Google have any trouble indexing sites that have a mobile version on a subdomain, for example, example.com and m.example.com. And then the question goes into, like, they’re seeing a lot of pages indexed without content.
So from our point of view, we don’t have well, at least as far as I know, we don’t have any problems with m dot domains in general, in the sense that this is one of the supported formats that we have for mobile websites. We don’t recommend the m-dot setup. So if you’re setting up a new website, I would try to avoid that as much as possible, and instead use a responsive setup. But it is something that can work.
So if you’re seeing this regularly with your website, that we’re not able to index your mobile content properly, then to me that would point more at an issue on your website, where when mobile Googlebot is trying to crawl, it’s not able to access everything as expected. So that’s kind of the direction I would head there to try to clean that up.
The one thing that throws people off sometimes with m dot domains is with mobile first indexing, we switch to the m dot version as the canonical URL. And it can happen that we show the m dot version in the desktop search results as well. So you also need to watch out for not only redirecting mobile users from the desktop to the mobile version, but also redirecting desktop users from the mobile to the desktop version.
And again, that’s something, if you have a responsive design setup, you don’t have to worry about that. So it’s like another reason to go responsive, if possible.