Data travel through a TCP/IP network in packets called datagrams. Each datagram is individually addressed with the following:
• The IP address of the host to which it should be delivered
• The protocol number of the transport protocol that should handle the packet after it is delivered to the host
• The port number of the service for which the data in the packet are bound
For data to be delivered correctly on a global scale, as it is on the Internet, the IP address must be globally unique, and the meaning of the protocol and port numbers must be well-known to all systems in the network. (Chapter 2 describes how the IP address is assigned to the network interface during the installation.) In the case of the IP address, you're the one responsible for making sure that it is unique. (In Chapter 13, "Troubleshooting," you'll see what kinds of problems occur when the IP address isn't unique.) Protocol and port numbers are different; they are defined by Internet standards. Thus, the protocol numbers and port numbers can be predefined in two files, /etc/protocols and /etc/services, that come with the Linux system.
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