I would save yourself the trouble and create a separate domain now for customer subdomains. The problem with your current path is that it is impossible to tell the difference between a 7sheep.net subdomain that is owned and operated by 7sheep and a subdomain that is owned and operated by a 3rd party.<p>For example, training.7sheep.net is an official subdomain, but I could create docs.7sheep.net and make it look like an official subdomain and request peoples account information or do other bad things. GitHub ran into the same problem when they started supporting GitHub pages. Originally these were subdomains off of github.com, but after all the spoofing and other issues they moved them all to github.io. This way you never need to create a list of 'reserved' names and don't need to worry about confusion down the road.<p>You can read about GitHub's transition and reasoning at <a href="https://github.com/blog/1452-new-github-pages-domain-github-io" rel="nofollow">https://github.com/blog/1452-new-github-pages-domain-github-...</a>.