How come network sniffing software can work over a switch?

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http://serverfault.com – We have several of standard non-managed 3com switches in a network. I thought switches were supposed to only send packages between peers of a connection. However it appears network sniffing software running on a computer attached to one any one of the switches is able to detect traffic (ie youtube video streaming, web pages) of other host computers attached to other switches on the network. Is this even possible or is the network thoroughly broken? Thanks. (HowTos)