This is true unfortunately. Most server administrators, myself included, will limit the amount of bandwidth the server gives out to its users to prevent them from saturating the WAN link. The server will then cope once the allotted bandwidth starts to "dry up" until it can't handle any more users. However with the advent of things like load balancing and clustering, we try to avoid knocking off unfortunate "end of the line" users
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Technically speaking, fiber optics do have "unlimited" capabilities if we had the equipment to harness it. Right now in the telco world we are limited to a single fiber running at ~40Gbps (OC-768) with 4 wavelengths of light multiplexed (DWDM). This is the same result as if we have 4 individual (~10Gbps) OC-192 lines. These limitations will hopefully disappear in the future since as of right now, these lines and the equipment to make use of them cost 10s of thousands of dollars.