At the transport layer, we add a port address, which is used to identify a specific application or process. Port numbers fall into three main groups:

• Well-known ports range between 1 – 1,023 and include protocols such as HTTP, DNS, and SMTP.
• Registered ports range between 1,024 – 49,151 and are assigned and used for specific services such as gaming applications, OpenVPN, and IPSec.
• Dynamic, private, or ephemeral ports are in the range of 49,152 – 65,535 and are not assigned to any specific application. They are used temporarily during a session, generally by the client.

When the transport layer header is applied to the data, a source and destination port are added. The type of port used depends on whether the packet is coming from the client or the server:

• If a client sends a packet, then the source port will be (in most cases) a randomly assigned dynamic or ephemeral port that is used, so when the server delivers a packet to the host, it uses that port to deliver the data.
• If a server sends a packet, then the source port will be either a well-known or a registered port.

The transport layer provides inter-host communication between endpoints. The following outlines a summary of the transport layer:

After the transport layer, the next layer is the network layer. As we'll see in the next section, the network layer is all about getting the data to the correct network.