PC hardware--CPU & Motherboard & RAM/Ethernet.
I know you can have a Network setup in a Star Topology or a Ring Topology or a Backbone Topology.
And every Topology will be diferant like 10Base 5 or 10Base 2 or 10Base T.
And they use Thinnet or Thicknet or Fiber Optic or U.T.P Cable.
And every Network can support diferant lengths of Cable.
I can not remember what lengths every Network can support.
So is a good rule to keep in my Head is this?
Never use a Cable longer then 50 Feet for PC to PC and even PC to HUB?
Twisted pair is specified for maximum run lengths of 100 meters (roughly 330 feet), 10BASE2 is specified to ~200 meters, and 10BASE5 to 500 meters. 100BASE-FX (optical fibre) has a specified maximum of 412 meters. Other optical solutions have different maximum run lengths - 10BASE-FL up to 2000 meters (2km), 100BASE-SX to 550 meters, and 100BASE-BX into the tens of kilometers. As far as easy memory devices, I don't have any for twisted pair or optical, but the 2 in 10BASE2 was meant to stand for "200 meters" (in reality you should consider the maximum more around 180 meters: http://en.wikipedia.org/wiki/10base2#Name_origination
) and 10BASE5 for "500 meters."
If dealing with hubs, also remember the 5-4-3 rule; http://en.wikipedia.org/wiki/5-4-3_rule
There are also other topology/connection considerations that have to be made for optical or thin/thicknet connections that differ from twisted pair - Wikipedia would be an easy place to start to familiarize yourself with those, or refer to the documentation of the equipment you're specifically working with.