INVITATION TO CYBERSECURITY 32 The physical layer manages the transmission and reception of physical signals over links. These signals include radio transmissions over the air (e.g., wireless networks), electromagnetic transmissions over wires, and light pulse transmissions over fiber optic cables. The link layer manages the interpretation of these physical signals as 1s and 0s on a per link basis. The most well-known link layer protocol is Ethernet invented by Robert Metcalfe in 1973. Most computers use either wired or wireless Ethernet to connect to the Internet. Ethernet addresses are forty-eight bits long and are called MAC (Media Access Control) addresses. NICs implement both the physical and link layers, and every NIC has a unique MAC address. There is no chance of running out since there are approximately 256 trillion of them! Table 2.6 The Internet’s five layer model. The OS handles the network and transport layers. These layers work together to manage the end-to-end connections between computers. The Transmission Control Protocol (TCP) and Internet Protocol (IP) were invented by Vint Cerf and Bob Kahn in the 1970s, and are the foundation of the Internet. They define how computer network data is addressed. By definition, every computer on the Internet has an IP address. IP addresses are like phone numbers. A phone cannot make or receive calls without a phone number, and a computer cannot send or receive data on the Internet without an IP address. IP addresses, like everything else in computers, are binary. They are thirty-two bits long and for convenience are written in dotted decimal notation as four numbers separated by periods ranging from [0-255] because each decimal number represents one byte of the IP address.7 Because they are thirty-two bits, there are approximately four billion IP addresses. Fortunately, this is not a limit on the number of devices that can connect to the Internet because multiple computers on a local network can share the same public IP address. For example, Internet service providers (ISPs) typically assign only one IP address to each home, but most households have multiple online devices. The one public IP address is assigned to the home’s router, and the router manages all the devices connected to it using private IP addresses. Private IP addresses are not globally unique, so mil- 7 This describes IP version 4 (IPv4) addresses. IPv6 is an updated version of the Internet Protocol, but IPv4 is still in common use.
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