I think this is probabgly something simple but im hosting a site on our work server which is visable to the outside world (ouside our firewall) using the web address e.g. http://www.domainname.com but i cant access inside the firewall but i can access it using the IP address its sitting on.

Any ideas?

Yes. The DNS request is resolving to an external IP and should be resolving to an internal IP, or you need to configure your router to NAT on packets inbound from the LAN port to simulate an external connection. This is a very common problem.