Hi i have a question... my final has got a very low grade and I can't explain it...I guess that the professor did some mistakes...
One question that weight a lot of points was about ethernet... the professor asked why ethernet is used only in local area network? resuming i wrote that ethernet is used only in LAN because it is shared and so for big network there could be a lot problem in using the shared channel.. for istance using protocol such as CSMA/CD, if a big network has a lot of hosts, they could wait a long time before sending...
is it right or am I wrong?

You have failed to mention any cable-length limitations, for one.

Additionally, you seem to be describing a single collision domain, which could be divided by switches and routers.

The first thing I thought of was cost. I've always wanted to implement fiber on my home network but fiber cards and switches are quite a bit more expensive than ethernet.

It is a goofy question. Lan is a statement that may be used to oppose wan (but not stated). In most cases Ethernet is not used to transmit over longer distances directly. CSU, T-1, wireless or fiber dedicated formats tend to be used over longer distances.

but the fact that ethernet is shared and so creates a lot of problems in a large network is completely wrong?

No, it is a well used and stable part of the world wide web and almost every computer on the planet in lans, wans, or any group in between.

Pretty much so. Ethernet is really no longer a shared environment. It was when 10Base2, 10Base5 and 10BaseT hubs were the prevalent hardware comprising a network. But with the advent of switches (where each port has a built-in bridge and therefore is a separate collision domain) you'd be hard pressed to find a collision anywhere in your network.

andrewthomas and jefro's comments on distance I think are the right answer.

(Ethernet is fast and reliable but has distance limitations. Ethernet won out over token ring on the cost issue, and over fiber, too as LVsFINEST mentioned but that wasn't the question.)

Copper based Ethernet used locally because it can only go 90-100m. But fiber based Ethernet can go much further based on fiber quality and module used, up to ~70km, no longer making it local... but it's still Ethernet.