In the IP address 128.18.72.24:80, what does the number 80 represent?

In the IP address notation 128.18.72.24:80, the number 80 indicates the TCP or UDP port number used for communication over the network. In this context, ports are integral to the transport layer of the OSI model, allowing multiple services to be run on a single IP address by directing traffic to the appropriate application based on the specified port.

When a client wants to connect to a server, it must know not only the IP address of the server but also which service it wants to communicate with. The port number provides this routing mechanism, directing packets to the right application. Typically, common services have well-known port numbers; for example, port 80 is primarily used for HTTP traffic, facilitating web browsing.

Other options reference elements of networking that do not relate directly to the port number's role in this address. For example, the IP address itself is the unique identifier for the device, the subnet mask defines the network portion of the IP address and aids in routing, and the network class categorizes IP addresses into ranges based on their leading bits, influencing how networks are segmented. None of these roles pertain to the function served by the number 80 in the provided IP address.