Normally you can access your domain name via either method but it all comes down to DNS records. You would normally have an A record for WWW which would point at the same location as DOMAIN.COM, which in theory is looking for a machine called WWW in your DOMAIN.COM or a SUBDOMAIN as used on I-WEB or any other CPANEL site. Therefore forum is a subdomain of .cableforum.co.uk
If you look at
www.dyndns.org they explain the wildcard approach a little better I.e. *.domain.com