When I login to my course at work, it gives me this error: "Cannot find server or DNS Error" even though my username/password worked fine. What is the problem?

Many employers and companies set up firewalls as security measures against hackers. Firewalls provide security by closing certain Internet ports, which disables student access to interactive computer applications like WebCT. In cases where students have been given permission to complete school work at their place of employment, employers have requested the port numbers required to modify their network and provide their employees with greater Internet access.