2. The Context of Cybersecurity: Cyberspace 35 packet has reached the destination computer. Port numbers are sixteen bits long and are written as decimal numbers ranging from [0-65,535]. Figure 2.11 Packet switching with routers shown as discs. All computers that connect to the Internet have a default gateway. The default gateway is the first hop for all of a computer’s Internet traffic. A hop is shorthand for a communication link, and when computers communicate with one another over the Internet, typically many hops are involved. Traceroute is a command line utility that shows the hops to a destination on the Internet (implemented as tracert in Windows). Traceroute works by sending three packets to each hop to determine the router name or IP address at that hop and calculate the roundtrip time. Figure 2.12 shows a traceroute from a client in Cedarville, Ohio, to a server located in Los Angeles, California. It shows that there are sixteen hops to the destination server, which means that the packets are received and processed by fifteen different routers along the way. The default gateway, i.e., the first hop, is 10.40.3.254. From trying to decipher the router names, it would appear the packets bounce around Ohio and then to Virginia, Georgia, and Texas before finally making it to California. This is not the most direct route, but, despite this, the last row of numbers show that the roundtrip time for the complete journey to Los Angeles and back is around eighty-three milliseconds (ms)—this is .083 seconds! This delay is called network latency and includes propagation delay (i.e., the amount of time it takes a signal to travel a distance) and processing delay (i.e., the amount of time it takes a router to receive, process, and resend a packet). To put this speed in perspective, it would take around seven hours to make a roundtrip flight from Cedarville to Los Angeles.
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