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Our home is on an island about 3 miles offshore. We use a wireless point-to-point link for Internet access.

Our ISP supplied all-in-one cable modem/router resides in an electrical box on the shore. There are no users here, and the only device plugged into the modem/router is the wireless bridge. The modem/router is in the default 'routed with nat' setting with DHCP enabled for the LAN. If required, I have the option to set it to 'routed without nat' or 'bridged', and can disable DHCP.

The ISP supplied all-in-one box gets it's WAN IP dynamically. The internal LAN IP is set to 172.16.0.1 and DHCP is enabled for the LAN starting at 172.16.0.10.

The bridges have static IP's of 172.16.0.2 and 172.16.0.3 with a gateway of 172.16.0.1.

3 miles away is the other wireless bridge that is connected to a switch. Also connected to the switch is a wifi access point (for wifi only devices) and various ethernet devices (NAS, printer, etc.).

The problem I have is that when the PTP link goes down (for various reasons) I also lose the DHCP server, meaning the LAN goes down.

I have a spare router at home (TP-Link Archer C7) that I can use to replace the switch and access point if need be, but I'm not sure how to configure it in conjunction with the shore-side router.

Do I leave the shore-side router set to 'routed with nat' but disable DHCP? How do I setup the home-side router? It would be great if I could still access the bridges from the LAN at home (if possible).

Any help offered would be greatly appreciated.

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Turn the antennas of both sides, 45 or 90 degrees in the x axis. Horizontal polarization is not a good idea traveling over water. Also it's a good idea to make sure you've got first order Fresnel zone which at 2.4 Ghz means antennas at both sides are 15 meters above water. For 5.8 Ghz, 10 meters. Minimum numbers are 10 and 6 meters, respectively. If there are obstacles, like boats, you need to add their hights. To get a stable link at 5 kilometers I would say a .45 meter dish is appropriate, at least on your side. Both sides if you are using 5 Ghz band which you should because of crowded 2.4 Ghz band.

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  • Hi Frank, any feedback regarding the questions I asked? I will deal with antenna realignment/positioning another time.
    – ron_jeremy
    Commented May 23, 2016 at 6:54
  • Well, I was too confused by your text and assumed you know what you're doing in the digital apartment. What your bridge on shore got for IPs and gateways is only relevant for change it's settings and when itself needs to connect to internet. Packets traveling though it shouldn't even know they are traveling through a bridge when the bridge use the same protocol on both sides, which this one does. Connecting the bridges together is not done at IP level, but SSID level. The only DHCP is the one on shore you explain, thus, losing DHCP is the same as losing Wifi link which was what I addressed.
    – Frank Sixteen
    Commented May 23, 2016 at 7:19
  • I guess this was not of much help if you are not into networking. Let's start at the bottom. Which of the bridges in your ptp link is master and which one is slave? The reason behind the question is to find out which one is responsible to keep the link up. Also to learn if you're familiar with important terms in this context.
    – Frank Sixteen
    Commented May 23, 2016 at 7:31
  • Welcome to Super User! Please read the question again carefully. Your answer does not answer the original question.
    – DavidPostill ♦
    Commented May 23, 2016 at 8:57
  • Oups, I thought I did answer just that one. Must have forgotten it. No you can not take any one AP to replace one in a ptp link. Such links are usually using proprietary protocols to access the APs even when it is in bridge mode. If you want to use your own equipment, you need an AP able to run as a client and an ordinary AP. Both may need to be in the class of outdoor router specially if 5 Ghz, because of the need of enough transmit power. You would need at least 15 dBm at both ends even with half meter dish.
    – Frank Sixteen
    Commented May 23, 2016 at 9:28

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