info on ip spoofing please

["Ian stuart Turnbull" <ian.t7 () hotmail co uk>]

Hello all,
At 
http://www.iss.net/security_center/advice/Underground/Hacking/Methods/Technical/Spoofing/default.htm

was this comment :-

QUOTE "
Examples of spoofing:

man-in-the-middle
packet sniffs on link between the two end points, and can therefore pretend 
to be one end of the connection "

My question is How can you sniff packets on a link that your machine is NOT 
on ie NOT on the same subnet??

Why am I at a loss to understand this. Is there a command/software that 
allows one to
say: sniff packets on port x of IP xxx.xxx.xxx.xxx ?

Please put me out of my agony on this.
Thanks for any info you can give.

Ian t

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