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:

Hôte Direct towards
forum.my_domain adress.my_forum

forumu subdomainde kullanmayı istersen DNS kontrö bölümüne CNAME kaydı girmen gerekirmiş. Cname url'eyi gözle görülmeyen biçimde yönlendirmeye yarar. Bu sayede forumunu subdomain.siteadi.com şeklinde yönlendirebilirsin.
TTL değeri yüksek tutarak dns yönetimi panelinde boş bir sub domain kullanabilirsin.
genelde "source", "origin", or "alias" bölümlerinde bulurmuş.

forumun orjinal urlsini buraya gir.
subdomain.siteadi.com u -> siteadi.com/adres/ şeklinde yönlendir.