host networking doesn't work

[xdaan]

hello all.. i have installed virtualbox 1.5.6_ose and created a linux guest (debian 4.0r3 netinstall) host kubuntu 8.04 with public ip.
networking is set through nat (by default) but it is not working...
when i run ifconfig it gives me the right configuration of nat and i can ping it, but i'm no able to ping the linux host (100% packet loss) and the outside internet (unable to find host www.google.com)

pinging from host to guest gives destination host unreachable...

i run vbox as a normal user (my login, member of vboxusers) and permissions of /dev/vboxdrv have set my username as a owner and group is "vboxusers"

what can i do when i want to bring the connection alive?