Virtual Box or Ubuntu?

[Gunawan]

Dear All,
I have install ubuntu hardy heron as guest operating system on my windows. I 
use Virtual Box with NAT.
Strange. I could ping to one external ip address for example my windows have 
192.168.11.100.
I could ping successfully to 192.168.11.50 inside my ubuntu but can get 
reply from another address.
for example I could not get reply from 192.168.11.55.
Any idea?
Regards,
Gun