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

In the IP address 128.18.72.24:80, the number 80 represents the TCP or UDP port number. In networking, a port number is used to differentiate between multiple services or applications running on the same host and allows the correct data to be sent to the appropriate application. Port 80 is commonly associated with HTTP, which is the protocol used for transferring web pages. This means that when a request is made to this IP address with port 80, it typically indicates that a web service is being sought.

The other options relate to different networking concepts. The network identifier typically refers to the portion of the IP address that identifies the network as opposed to individual devices. The subnet mask is a separate number used to determine which part of an IP address belongs to the network and which part can be used for hosts. The IP version signifies which version of the Internet Protocol is being used, with common versions being IPv4 and IPv6. Therefore, the identification of port number 80 in this context is crucial for understanding how data is directed to web services over the internet.