What is the role of transport layer port numbers in firewall checks?

• A. They help determine which packets are allowed or denied access to the network
• B. They encrypt data
• C. They define user permissions
• D. They identify IP addresses.

Answer: (C)

Port numbers at the transport layer identify which service or application a packet is intended for on the destination host. Firewalls use these numbers to decide whether to allow or block traffic based on the service being accessed. For example, traffic targeting well-known ports like 80 (HTTP) or 443 (HTTPS) is commonly permitted, while access to other ports may be denied. This role is distinct from encryption (which is about protecting data in transit) and from user permissions (which are about who is allowed to do what). IP addresses locate the endpoints, while port numbers point to the specific service on those endpoints. In practice, many firewalls also track connection state, using the combination of IPs and ports to manage and validate ongoing communications.