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I have an ISP provided modem 192.168.1.1 with DHCP on, then a Linksys router LAN connected on with ip 192.168.16 also with DHCP.

Then a Synology 10.172.1.120 as it is connected to the Linksys router (10.172.1.1)

I want the Synology 10.172.1.120 to talk and see my outdoor the IP camera 192.168.1.15 that is connected to the ISP modem.

I cannot connect the camera to the router or the sinology to the modem, they need as they go to different rooms with internal cables. I don't want to set the Linksys router in bridge mode or turn off DCHP here. I enjoy the security and features of the Linksys.

I tried to turn off the DCHP on the ISP modem but I couldn't get online.

is there a way to forward ports or have some settings where devices in two different IP pools can see and talk to each other?

  • Please edit your question to provide a network map and correct the IP address of the Linksys router in your first sentence. – Twisty Impersonator Oct 29 '17 at 21:44
  • This is more difficult than it sounds because you have a routing issue. Not only will it involve port forwarding, it will involve static routes configured on the ISP subnet. – Appleoddity Oct 29 '17 at 22:48
  • @Appleoddity - while I agree that this is complex because of routing issues, it does not neccessarily require port forwarding or changes to static routes on the ISP subnet. – davidgo Oct 30 '17 at 4:06
  • @davidgo you just provided an answer using static routes on the ISP subnet. :) – Appleoddity Oct 30 '17 at 12:01
  • @appleodity - you might want to revisit your idea of "the ISP subnet". My answer referred purely to IP addresses in RFC1918 space (ie 10.x, 192.168.x) which are not routable to an ISP, and thus are not on the ISP subnet. (Traffic leaving a WAN Interface is not at all synonymous with an ISP subnet, and nothing I advocated operated on the WAN Interface connected to the ISP.) – davidgo Oct 30 '17 at 18:37
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In your question you have excluded the simplest and typically best way of setting this up - ie flattening the network by bridging everything.

What you need to do (in general terms) is

  • Assign a static WAN address to the linksys router (on 192.168.1 subnet)

  • disable NAT on the Linksys router

  • Add static routes with the Linksys 10.172 on the ISP router, specifying the Linksys gateway (192.168.1.X).

  • Considering this is likely home equipment, I can’t say I’ve seen a home router that supports disabling NAT, although I’m sure they exist. Second you’ve created an asymmetric routing issue by hairpinning the route on the ISP equipment. Lastly, I wouldn’t be surprised if the ISP equipment additionally doesn’t support static routes - although, again, I’m sure they exist. So in theory it works, but in practice it’s problematic. I’m not here to beat up the answer, but there is nothing done here technically called “bridging.” – Appleoddity Oct 30 '17 at 11:59
  • @Appleoddity Ouch - We agree on almost nothing! I'm not aware of any routers which don't support disabling NAT. My standard ISP provided router and all *wrt routers support this. NAT can be disabled on most/all Linksys routers by going to "Advanced Routing", and disabling it in the "Setup" tab. This functionality has been baked into ?all? Netgear routers since forever as well. – davidgo Oct 30 '17 at 18:47
  • I have not created an asymetric routing issue. This occurs where packets leave 1 interface and come back a different one. In this case the default route pushes traffic out the correct interface. (I did consider adding routes for it, but it needlessly complicates things). In the 20 years I've been using routers, I've not seen a router which does not support adding static routes - I remember doing it on cheap-as AI agree that what I'm doing is not bridging - I just commented that flattening the network by bridging everything is probably a better way of setting it up. – davidgo Oct 30 '17 at 18:50
  • I'm always open to a discussion, something we can take in to chat. But, try and try as I may, I haven't been able to find a document confirming NAT can be disabled on any home routers - including Netgear and Linksys. I'd be happy to take a look if you can provide a specific model and page in the manual that says NAT can be disabled. For instance, here is one search result on the topic: community.netgear.com/t5/R6000-Series-AC-WiFi-Routers/… – Appleoddity Oct 30 '17 at 18:56
  • In addition, your description of asymmetric routing is not accurate. Asymmetric routing is when a source packet traverses one path towards a destination, and on the return trip traverses a different path. In this case, a packet from ISP subnet goes to default gateway then back to Linksys. A return packet will go straight from Linksys to the machine on the ISP subnet. Please read more, maybe here: my.stonesoft.com/support/document.do?docid=1377 it doesn't have anything to do with interfaces. – Appleoddity Oct 30 '17 at 18:58

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