Hey guys I hope you can help

I have apache set up an my network at home and I am using cable internet. If i type in http://ipaddress/ i can see my page
So this would mean that AT&T , my ISP didnot block port 80. So now i am trying to use Dynamic Dns. I set
my domain name to my ipaddress, but it is not working. However if I :
ping domainname.com
I get a response from the correct IP address. So why is it not working from the browser?
I really would appreciate your response

PeAcE