What do Layer 4 addresses refer to?

Layer 4 addresses refer specifically to port numbers used in the Transport layer of the OSI model. In networking, the Transport layer is responsible for end-to-end communication and for establishing, managing, and terminating connections. Port numbers help distinguish different applications or services running on a host by associating specific applications with their transport layer protocols, like TCP or UDP.

For instance, when a web server communicates over the internet, it typically uses port 80 for HTTP or port 443 for HTTPS. By using port numbers, data can be directed to the correct application upon reaching its destination. This is essential for multiplexing multiple connections over a single IP address, ensuring that data is sent to the appropriate process running on the device.

Other options, while related to addressing in networks, do not pertain to Layer 4 specifically. IP addresses operate at Layer 3 and are used for routing packets between hosts, MAC addresses function at Layer 2 for local network communications identifying devices on the same network segment, and network addresses don't define a specific layer in the OSI model, but rather can refer to several types of addresses used throughout networking.