There have been many times I have needed to run NTOP on a Debian host and got it working, but never set it up 100% correctly. Usually I'd get NTOP working and configured and by that time I was too anxious to wrap it up. I'd modify /etc/rc.local and bring up the interface with a simple ifconfig eth1 up.
Today I had to reboot an NTOP host and ifconfig wasn't already in rc.local so I decided to do it correctly. I found a blog post here. I wouldn't have re-written another blog post, but he doesn't allow comments on his.
Modify the file /etc/network/interfaces with the following content:
# For NTOP
auto eth1
iface eth1 inet manual
up ifconfig eth1 up
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