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I have two routers, that I want to represent two subnets. They are configured in this way:

Primary router (from ISP):

  • LAN: 192.168.0.1 / 24
  • DHCP: 192.168.0.10-100
  • SSID: 'router1'

Secondary router:

  • IP address: 192.168.0.101 (also tried with DHCP)
  • Gateway: 192.168.0.1
  • LAN: 192.168.2.1 / 24
  • DHCP: 192.168.2.100-200
  • SSID: 'router2'
  • cable goes from this router's WAN to primary one's LAN port.

Now, I have two laptops. One connected through WiFi other by cable, but both to Primary router.

WiFi Laptop can access Web fine, I can also get to admin panel of the primary router.

Ethernet laptop can't make it to the web. arp -a table shows only 192.168.2.1 which should be the default gateway of devices in the Secondary Subnet. I also get assigned 192.168.2.113. The problem disappears as soon as I disconnect cable and connect wirelessly.

Why is that so? I'm connected straight to the primary router.

  • 1
    That shouldn't happen if things are setup as you describe, is the problem laptop getting new settings when it's connected, or could they be old settings from when it was last connected to the secondary router? Which wifi network does it connect to with no problems, primary/router1 or secondary/router2? Is either router in a bridged or wds mode? – Xen2050 Dec 21 '17 at 22:13
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Either the router from your ISP is not configured to permit the LAN subnet on router 2 through NAT or you are performing double NAT (NAT on both ISP and router 2), or both.

Double check your ethernet connection as well, if your laptop ethernet is getting an IP from router 2, it is likely not connected to the ISP router.

  • What would be an example configuration of such NAT? Regarding the cable, I'm 200% sure it is connected to the Router 1. – adamczi Dec 21 '17 at 18:47
  • Unfortunately, I couldn't answer that because there are so many different router types and many ISPs do not allow you to fiddle with NAT on that level. It should not be possible for the ISP router to hand out DHCP from router 2 unless there is a cabling problem. Check to see if you are connecting the LAN of ISP to the LAN of router 2, if so, change it so that the WAN of router 2 is connected to the LAN of the ISP router. – Ray Dec 21 '17 at 18:48
  • My ISP router is Thomson TWG-870 and has super limited functionalities, guess I can't do it there. Thanks for the tip about LAN-LAN. This might be an issue, but right now I was told no disruptions are allowed, so I have to come back and check it tomorrow. – adamczi Dec 21 '17 at 18:57
  • Got it. Quite embarassing, I haven't had access to the room with other router yesterday (restricted), today I found out somebody connected routers using LAN-LAN connection... I didn't even think someone would do it if they need separate subnet, but well. Lessons learned, better check than assume things. Thanks @Ray. – adamczi Dec 22 '17 at 10:47
  • Interesting packets were routed through Primary to the Secondary router and then ended/stopped - but only using cable. On WiFi it was correct. – adamczi Dec 22 '17 at 10:48

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