A typical Ethernet LAN uses unshielded twisted-pair (UTP) cables with RJ-45 connectors (which are slightly bigger than telephone RJ-11 connectors). Because these cables have only two ends, you need an intermediary device to connect more than two computers. That device is a hub.
A hub works at Layer 1 and connects multiple devices so that they are logically all on one LAN.
Physical Interfaces and Ports
The physical connection point on a network device—a hub, switch, or router—is called an interface or a port.
Don't confuse this definition of port with the application layer port numbers discussed in the "TCP/IP Transport Layer Protocols" section later in this chapter.
A hub has no intelligence—it sends all data received on any port to all the other ports. Consequently, devices connected through a hub receive everything that the other devices send, whether or not it was meant for them. This is analogous to being in a room with lots of people— if you speak, everyone can hear you. If more than one person speaks at a time, everyone just hears noise.
All devices connected to a hub are in one collision domain and one broadcast domain.
NOTE A hub just repeats all the data received on any port to all the other ports; thus, hubs are also known as repeaters.
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