When you choose the sub-domain which you wish to use on your forum (forum.my_domain, for example, is a sub-domain of my_domain), you have to create the corresponding CNAME entrance in the DNS administrator of your domain: a CNAME entrance allows to point an url towards the other one in a transparent way. in this way, you can link forum.my_domain to adress.my_forum.
Any recorder of domains names possesses different interfaces, and thus different methods to create such CNAME.
However, in a general way, the process takes place as follows:

• Connect to your user account on the site of your domain name provider, and go to the DNS management.
• Check that the chosen sub-domain is not already used.
• In the generally entitled field "source", "origin", or "alias" enter the prefix of sub-domain (i.e. "Forum" if you want to use the sub-domain forum.my_domain).
• In the generally entitled field "destination" or " target host ", enter the original url of your forum, adress.my_forum.
• You will maybe have to inform the TTL (Time To Live): give the maximal possible value.

After validation, you should obtain a line of this type:

HôteDirect towards forum.my_domain adress.my_forum
Of course, we invite you to consult FAQs, even to contact your host support.