hi colinjones,
i know the core is also a router and i read the Network_Setup Wiki Page. The reason why i must use two different networks is: my core isn't online everytime, so i have to use my wlan-router for my external network, so all the laptops (wife's laptop also) have internet - also when the core is offline...
By the way, i have managed the routes and everything works now as expected. thank you to everyone.
avajon