What is the purpose of port numbers in the Transport Layer?

What is the purpose of port numbers in the Transport Layer?

• A. To direct network traffic to the correct application.
• B. To determine the route through the network.
• C. To enforce security policies.
• D. To identify hosts on the local network.

Answer: (A)

Port numbers in the Transport Layer serve as the addressing mechanism that allows a host with a single IP address to run many applications simultaneously. When a Transport Layer segment arrives at a host, the destination port tells the protocol (TCP/UDP) which application process should receive the data. This demultiplexing lets, for example, a web server listening on port 80 or 443 receive HTTP/HTTPS requests, while a mail service uses its own port, all on the same machine. The network route is determined by IP addresses and routers, not by port numbers, so port numbers don't influence the path through the network. They also don't identify hosts on the network—the IP address does that—and they aren't security policies, though firewalls may filter traffic by port. In short, port numbers direct traffic to the correct application.

Port numbers in the Transport Layer serve as the addressing mechanism that allows a host with a single IP address to run many applications simultaneously. When a Transport Layer segment arrives at a host, the destination port tells the protocol (TCP/UDP) which application process should receive the data. This demultiplexing lets, for example, a web server listening on port 80 or 443 receive HTTP/HTTPS requests, while a mail service uses its own port, all on the same machine. The network route is determined by IP addresses and routers, not by port numbers, so port numbers don't influence the path through the network. They also don't identify hosts on the network—the IP address does that—and they aren't security policies, though firewalls may filter traffic by port. In short, port numbers direct traffic to the correct application.