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Part: V Appendices > How Ports Complete the Connection

How Ports Complete the Connection

So far you have learned that IP addresses are used to make sure that data gets delivered to the correct destination. However, since the IP address only identifies the computer network card, then how does the computer know what application the data it receives is intended for? For example, let's use the postal service as an example again. You can send letters simply to an address but without putting a person's name on the envelope; then when the letter gets delivered, who gets to open it?

Ports are the solution to this problem in the TCP/IP suite. In addition to an IP address, a port number is used to identify the source and the recipient of data transmitted using TCP/IP. However, IP doesn't have fields in its header to support port numbers, it only handles the source and destination computer IP addresses. It is the higher-level protocols such as TCP and UDP that use port numbers.


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