Netopia 435 Reference Manual page 155

7-22 Netopia ISDN Router Reference Guide
Input and output filters—source and destination
There are two kinds of filters you can add to a filter set: input and
output. Input filters check packets received from the Internet,
destined for your network. Output filters check packets transmitted
from your network to the Internet.
packet
input filter
LAN
WAN
packet
output filter
The Netopia ISDN Router
Packets in the Netopia ISDN Router pass through an input filter if they originate
in the WAN and through an output filter if they're being sent out to the WAN.
The process for adding input and output filters is exactly the same.
The main difference between the two involves their reference to
source and destination. From the perspective of an input filter, your
local network is the destination of the packets it checks, and the
remote network is their source. From the perspective of an output
filter, your local network is the source of the packets, and the remote
network is their destination.
Type of filter "source" means "destination" means
Input filter the remote network the local network
Output filter the local network the remote network