how is udev integrated in fedora or redhat?

Hi,

I have some question about the network device naming mechanism in redhat&fedora.
On my box with 2 on-board network interface and another 2 intel
network interface,the order of these four interfaces will change from
time to time.I mean sometimes the 2 on-board are eth0 eth1,sometimes
they are eth1 eth2.

I know I can use MAC address to fix the order.I'm just curious about
how could the order change boot by boot without any modification on
configuration or hardware.

could any one give some suggestion?

Thanks
scott