2. The Context of Cybersecurity: Cyberspace 33 lions of computers are able to use the same IPs at the same time, greatly multiplying the number of devices that are able to connect to the Internet to way beyond four billion. The IP address of Cedarville University’s website (www.cedarville.edu) written in dotted decimal notation is 163.11.75.44. Written in binary it is: 10100011000010110100101100101100 As a decimal number, this value is 2,735,426,348, and as a hex number, it is 0xa30b4b2c. Most web browsers can use multiple different IP formats to “dial the web server’s number.” For example, all of the following links go to the same webpage (type them into your web browser’s address bar see!): http://www.cedarville.edu http://163.11.75.44 http://2735426348 http://0xa30b4b2c 2.4.2 Packet Switching “Go West, young man.” - Horace Greeley Sending data over the Internet is analogous to mailing a letter via the United States Postal Service (USPS). If Alice wants to send a letter to Bob, she writes a message on a piece of paper, sticks the paper in an envelope, writes Bob’s name and mailing address on the front of the envelope, and drops it in the mail (let’s forget about the stamp—Internet traffic requires no postage!). From there, the USPS takes over, and the address is dissected first by state and zip, then by city, road, and eventually house number as the envelope makes its way through the postal service from Alice’s house to Bob’s house. Once it reaches Bob’s house, the name on the envelope is the last part of the address needed to make sure that Bob opens the letter and not another member of his household. When computers send messages over the Internet, they address them with something like an envelope called a header. On the Internet, a header is a prefix added to a packet that specifies delivery-related information. The non-header part of the message data is called the payload. The IP header contains a source and destination IP address as well as some other fields. The protocols at each layer add their own header to communicate with their peer protocol at the destination computer. When the packet arrives at the destination and is processed, the header is stripped off and the payload is passed up the stack to the next layer—just like the envelope is thrown in the trash after a letter has reached its destination. Data is sent over the Internet in packets, and the computing devices that process and route packets towards their destinations are called routers. Routers make up the core of the Internet and are the basis of the network layer. The network layer is in charge of
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