An old friend of mine is a native of San Francisco and still
lives there. The traffic is horrible there, in part due to the need for lots of
people to commute across the San Francisco Bay. There simply aren't enough
bridges for the number of cars that need to cross the Bay. His solution: Fill in
the Bay with dirt and pave the whole thing. Then there would be plenty of roads
to allow people to cross the Bay.
If you tried to connect 10 PCs in a network, using a cross-over
cable between NICs (as shown in figure 4-10), you would begin to do the
equivalent of paving the San Francisco Bay. You know how two computers can use
Ethernet NICs with a cross-over cable to communicate. However, to connect to
other PCs, you would need more Ethernet NICs, and your PC probably does not have
enough room for all the cards. Also, you would need to run cables between your
PC and all the other PCs, or at least get the electrician to run the cables. If
you tried to do this for 100 PCs on the same floor of the building, and every PC
wanted to connect to every other, you would have 99 cables connected to 99 NICs
inside each PC!
The alternative to running a cable to every other PC is to run
a cable from each PC to a wiring closet and connect the cables to a networking
device, called an Ethernet hub. An Ethernet hub
provides several functions, but mainly it allows the electrician to cable each
device to the hub using only a single NIC and single cable, eliminating the
cabling problem. The hub simply listens for incoming electrical signals, and
when received, the hub repeats the same electrical signal to every other device
that's connected to the hub. Figure 4-11
shows the basic operation.
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