The reason for blocking ports from the normal traffic is the access control. Firstly, firewalls used to block ports is to defend a network parties which is trying to access the internal service from outside. The unused and unsafe ports should be closed when they are not in use in case of the attacks through these ports. The blocking service prevents the DNS Spoofing, which is an attack to return an incorrect IP address as the response. Secondly, the firewalls restrict the service that are deemed unsafe from inside to outside. Blocking outbound traffic could limit the attackers’ actions once they are a composition if the network. The service prevents the ARP Cache Poisoning which could result in information inspection, information modification
