What configuration options does your modem/router give you? By default many are set up that the default route for all traffic on the LAN ports is outbound.

Which means they do get encapsulated up and spat over ATM to the ISP for onward forwrding.

Easy to turn off though - just change the default route for those other ports.
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Rory
MkIIa, blue lit buttons, memory upgrade, 1Tb in Subaru Forester STi
MkII, 240Gb in Mark Lord dock
MkII, 80Gb SSD in dock