Port Blocking and Network Users
Before you implement port blocking, you should have a very good idea of what the port is used for. Although it is true that blocking unused ports does not have any impact on internal network users, if the wrong port is blocked, you can create connectivity issues for users on the network.
For instance, imagine that a network administrator was given the task of reducing the amount of spam emails received by his company. He decided to block port 25, the port used by the Simple Mail Transfer Protocol (SMTP). He may have succeeded in blocking the spam email, but in the process, he also prevented users from sending email.