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CHAPTER 3 Overview of the Internet > 3.6 Further Network Facilities - Pg. 116

116 CHAPTER 3 Overview of the Internet FIGURE 3.32 Ethernet information for frame 1210 from the UDP boids demo Bits 0­31 32­63 64­95 96­127 ... ... 0 15 16 Source MAC Address ... Data Data 31 Destination MAC Address ... ... Destination MAC Address EtherType ... Source MAC Address CRC Checksum FIGURE 3.33 Ethernet Type II frame but it is probably removed at a low level by the Ethernet device itself. At the phys- ical level, there are other bits that are sent as preamble, but we needn't be con- cerned with those. Despite all the protocols we've discussed before, there is still a small piece miss- ing. After all the encapsulation, how does a machine on an Ethernet know what the destination MAC address is? This is the role of the Address Resolution Protocol (ARP). We fleetingly saw an example in Figure 3.3. ARP is a link-layer protocol which allows hosts to identify which MAC address is mapped to a particular IP address . 3.5.2 Comparisons To wrap up the section on link and physical layers, in Table 3.3 we summarize some key features of each technology: bandwidth, latency and range. There are many vari- ations on these technologies, so this should be used as a rough guide. 3.6 FURTHER NETWORK FACILITIES So far we've discussed the Internet as a point-to-point service supporting a best effort service. Of course the Internet has been extremely successful, but many applications don't map easily to these conditions. There have been many efforts to extend the