Hi
The Router's NAT firewall affect only Traffic coming in from the Internet.
The Router is a combo unit of Routing Circuit, and a switch in the same box.
LAN traffic is only using the switch.
The difference that you experience could be a result of different setting
other Router's DHCP (Computers' IP assignment).
There are differences between Routers in the way they handle the IP
assignment. Some Routers keep the same IPs according the computer MAC
number.
Others can assigned it in up hazard manner (I.e. giving different IP number
to the same computer each time it starts.
The latter can confuse the LAN Traffic. The best is to use a Router that can
do Static DHCP. I.e the computer's TCP/IP can be set-to Auto Obtain and they
always would get the same IP.
Jack (MS, MVP-Networking).

"Stephen Ford" wrote in message
...
2-PC LAN + network printer + router

Win XP Pro SP3

Can Windows Firewall affect / stop LAN traffic? The following problem
often raises questions about the firewall and I want to understand its
relevance. Discussions seems to get very convoluted and end up down blind
alleys.

I contend that Windows Firewall (WF) only affects WAN traffic but I
suspect that just by replacing router#1 with router#2, this does not
result in the WF stopping PC-PC ping traffic. Is that right?



I am trying to replace a router with a new one. With router #1 in place
Ping between the PCs works fine. With router#2 in place all network
traffic is fine (eg PC-printer, PC-router & PC-Internet) apart from Ping
PC-PC.

I have been through the usual issues of network configuration issues
ad-nausium includin IP addresses, DNS, sub-net mask etc. In deed if there
was a problem, then nothing would work with router#1 in place. But with
router#1 in place all network comms is fine. Put router#2 in place and all
network traffic is fine apart from ping PC-PC.