There are several approaches of how to virtualize networking for containers. Those are desribed below.
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Layer 3 virtualized network interface
This one is employed by OpenVZ's venet. See venet.
Layer 2 virtualized network interface
This one is employed by OpenVZ's veth. See veth.
Layer 3 isolated network (bind filtering)
This one is implemented in Linux-VServer. Basically, when a container calls bind() with INADDR_ANY, kernel actually binds the socket to some specific IP address(es). Some more details (not much) can be found at http://linux-vserver.org/Paper#Network_Separation