Traveling by air these days is a lengthy adventure. Check-in procedures, protocols and security checks often take you hours. The email you send out each day goes through similar procedures, yet an email's entire journey takes only seconds.

When you send an email, the information from the sender is delivered through your Internet connection in tiny pieces. These conveniently transferable chunks are called packets. Packets are small, from 20 bytes to about 1500 bytes. Packets must be small to allow many to share network lines efficiently. One typed character takes up one computer byte of computer storage/transfer space. Even the smallest email message is broken down into dozens of packets. Large transfers—such as clicking a link to view a webpage—require thousands of packets. Packets deliver different chunks of the message by traveling to the same destination independently, pursuing the most convenient route possible at the moment of transmission. This means packets almost always split up as they race their way toward the destination. (The first sentence of your email comes in one packet routed through Minneapolis. The next sentence comes in a separate packet routed through Denver.)

Your email is treated like a massive jigsaw puzzle, but without the box. You put the puzzle in whole when you create the email, then your computer operating system breaks the puzzle into pieces for delivery. Instead of a box to keep the puzzle pieces together, a code number is written on the top of each piece so that all the pieces are individually labeled with the correct destination and numbered so that the receiving computer can reassemble them easily.

Each packet (puzzle piece) has three parts: the header, the payload (often called the body or the data), and a trailer (often called the footer). The header includes the email's origin and destination addresses. The header also contains the code number that allows packets to reunite when the journey is complete. The payload is the actual data being sent, the text of the email. Some routers require the payload to be a specific size. When this happens the remaining space will automatically be padded with blank information to make the packet match the size requirement. This requirement keeps data uniform so that if data is lost, the loss is easy to detect. The trailer of the packet tells the receiver that it has reached the end of that piece. Some trailer packets also have built-in error checking that make sure the packet data has arrived intact.

Firewalls monitor data packets with tools called packet filters. The packet filter analyzes incoming and outgoing packets, partially reassembling the puzzle to check the contents. Having someone plant a bomb in your luggage at the airport is rare, but receiving malicious programs via email is far more common. If this happens, a firewall packet filter can detect trouble and block the receiving area to protect your computer from harm. Like a vigilant airline security barrier, a firewall guards entry into your system, allowing only acceptable data to pass. When shopping for a firewall program, make sure the firewall has packet-filtering features and the ability to actively monitor your system ports.

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