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NAT puzzle

mmesford

Junior Member
Dec 26, 2016
9
0
6
I'm looking for troubleshooting ideas. The setup is a cable modem connected to a Cisco RV180 router. There is a switch connected to one router port and two PLCs connected to the switch. Each PLC has an embedded web server. I can connect a laptop to the switch, or to a router port, and call up the webpage of either PLC. I've configured the router's firewall to allow traffic on port 80 and to forward it to one PLC. The puzzle is that PLC A (192.168.1.5) is accessible from outside the network, as expected, when 192.168.1.5 is entered in the NAT field in the firewall page of the router setup screen. But when I change it to 192.168.1.8, the IP of the other PLC, it's not accessible from outside the network. I can ping it from outside the network but it won't respond to requests from a browser. And, as I mentioned, it does respond to browser requests from within the network. It seems like it has to be something in the router but I can't think what to try. The page on the PLC that does work is much simpler and smaller than the page on the PLC that won't connect but I can't think how that would make a difference.
 

sdifox

No Lifer
Sep 30, 2005
86,017
9,912
126
generally speaking when you setup port forward, all traffic to a particular port goes to one destination internal ip. so your external ip port 80 can be mapped to plc1 or plc2 but not both. you just need to map say external port 2000 for plc2 and specify external ip:2000 when you ewant to access plc2.
 

mmesford

Junior Member
Dec 26, 2016
9
0
6
Sorry, I wasn't clear. I only want to access the one PLC from outside. I've been mapping to the other one just for troubleshooting purposes. I change the mapping each time to test, so there's only one PLC mapped at any given point in time. When mapped to PLC1 everything works as expected. When mapped to PLC2 I can't access it from outside the local network. Weird.
 

sdifox

No Lifer
Sep 30, 2005
86,017
9,912
126
Sorry, I wasn't clear. I only want to access the one PLC from outside. I've been mapping to the other one just for troubleshooting purposes. I change the mapping each time to test, so there's only one PLC mapped at any given point in time. When mapped to PLC1 everything works as expected. When mapped to PLC2 I can't access it from outside the local network. Weird.

did you reset the state table after you change the mapping?
 

mmesford

Junior Member
Dec 26, 2016
9
0
6
No. I'm not familiar with that term. I've tried rebooting the router and the cable modem.
 

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