Ethernet is a technologyusedto connect upto a few hundred computersanddevices.The connections
use wires of up to 100m or ﬁbers of up to a few km. The bit rate on these wires and ﬁbers is usually
100Mbps but can go to 1Gbps or even 10Gbps.The vast majority of computers on the Internet are
attached to an Ethernet network or, increasingly, its wireless counterpart WiFi. We discuss WiFi in
a separate chapter.
We ﬁrst review a typical Ethernet network.We then explore the history of Ethernet. We then
explain the addressing and frame structure. After a brief discussion of the physical layer, we examine
switched Ethernet. We then discuss Aloha and hub-based Ethernet.
3.1 TYPICAL INSTALLATION
Figure 3.1 shows a typical installation.The computers are attached to a hub or to a switch with wires
or ﬁbers. We discuss hubs and switches later in this chapter. We call ports the different attachments
of a switch or a hub (same terminology as for routers). A small switch or hub has 4 ports; a large
switch has 48 or more ports. The hub is used less commonly now because of the superiority of the
switch. The ﬁgure also shows a wireless extension of the network.
Figure 3.1: Typical Ethernet installation.