Explain the concept of port numbers in the context of the transport layer. How are they used for addressing and multiplexing?

Port numbers in TCP and UDP are used to differentiate between multiple communication endpoints (processes) running on the same host. They are 16-bit unsigned integers ranging from 0 to 65535. Port numbers below 1024 are reserved for well-known services (e.g., HTTP uses port 80, SMTP uses port 25). Port numbers above 1023 are available for use by applications and are often dynamically assigned. Port numbers, along with IP addresses, are used for addressing and multiplexing, allowing multiple network services to operate simultaneously on a single host.