Source : Free On-Line Dictionary of Computing

RG8
     
         (Or "thicknet" - "thick
        {Ethernet}") The original "full spec" cable, used for
        {10base5} {Ethernet} networks.  RG8 is stiff, large diameter
        {coaxial} cable with an {impedance} of 50 ohms, a member of
        the "Radio Guide" series.  The outer sheath is usually yellow,
        to indicate double shielding, so it is often just called
        "yellow cable".
     
        10base5 cable is designed to allow transceivers to be added
        while existing connections are live.  This is achieved using a
        "vampire tap" - a device which (with sufficient practise)
        clamps onto the cable, forcing a spike through the outer
        shielding to contact the inner conductor while other spikes
        bite into the outer conductor.  This is often built into the
        transceiver and a more flexible multi-wire cable carries the
        connection between the transceiver and the {node}.
     
        {RG58} is a cheaper, more flexible alternative.
     
        (2002-06-17)