When a host receives a packet (unit of network communication), how does it know which process to send the packet to?

When a host receives a packet, it determines which process to send it to based on the information contained in the packet's header. Here's a step-by-step breakdown:

1. The packet arrives at the host and is intercepted by the network interface card (NIC). The NIC reads the destination IP address in the packet header to determine if the packet is intended for this host.

2. If the packet is intended for this host, the NIC sends the packet to the host's operating system.

3. The operating system then looks at the transport layer protocol (usually TCP or UDP) in the packet header. This protocol determines how the packet is processed.

4. The operating system then looks at the destination port number in the packet header. This port number is associated with a specific process running on the host.

5. The operating system sends the packet to the process associated with the destination port number.

6. If the process is running and ready to receive the packet, it will accept it. If not, the packet may be queued for later delivery, or it may be discarded.

This process ensures that each packet is delivered to the correct process on the host, allowing multiple processes to share the network interface.