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Protecting Your Data: A Guide to Windows Firewall
Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)
Once data finds its way to your computer using your IP address, the trip isn't quite over. IP uses Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) to help direct the network communication into the appropriate application on your desktop.
Using our previous example of the postal service, the TCP and UDP protocols are similar to the name used in the destination address on an envelope. Although many pieces of mail may arrive at a single location, not all of the envelopes may be for one person. Each envelope includes a name to help guide the mail to the correct recipient.
TCP and UDP are used to guide the network traffic to the correct application on your computer. If a web browser and an email program are being used simultaneously, the network traffic must know which program should receive which pieces of data. A web browser wouldn't know what to do with email data, and an email application can't do anything with web browser traffic!
Don't be thrown over the odd names associated with these protocols. The important thing to remember is that TCP and UDP help guide the traffic from the Internet into the correct web browser screen or email program.
Instead of using names on an envelope to identify and separate applications, TCP and UDP use a series of numbers called "ports." When network communication between two stations begins, the two stations decide which ports will be used for the conversation. When the conversation is over, the ports are not used for further conversations. Any additional conversations must use a different pair of ports to communicate.
