Wikipedia:Reference desk/Archives/Computing/2022 January 6

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January 6

Router routing mystery

Dear Wikipedians:

My home Internet is provided by a DSL modem/router combo device, all the devices plugged into the DSL modem/router combo get IP addresses on the subnet 192.168.2.x.

However, the Wifi provided by the DSL modem/router combo device is aged and unreliable and drops from time to time, also the number of Ethernet ports provided by the DSL modem/router combo are almost exhausted (there are only 4 Ethernet ports as is standard, and VoIP phone boxes and desktop computers quickly take over all the ports). Therefore I bought a brand new ASUS Wifi-6 router a few months ago, and plugged it into the DSL modem/router combo device. The new ASUS router is meant to provide solid Wifi for the entire house, and also provide a few more spare Ethernet ports for more landline devices (such as IPTV set-top boxes). All devices plugged into the new ASUS router get IP addresses on the subnet 192.168.50.x. (and so are clearly on a different subnet from the 192.168.2.x that devices plugged into the DSL modem/router combo device are on). The ASUS router itself has the IP address 192.168.2.111 on the DSL modem/router combo device network (designated "WAN" on the ASUS router), while on its own network it is the gateway device with IP address 192.168.50.1.

However, I have discovered the following curious phenomenon: from a 192.168.50.x device, such as my ultrabook connected to the ASUS router via Wifi, I am able to access *all* devices on the 192.168.2.x subnet. However, if I tried the reverse, nothing works. From my 192.168.2.x desktop computer, I cannot access any devices on the 192.168.50.x subnet. Granted, I can set up port-forwarding on the ASUS router, but that's another subject entirely.

What I don't get is why isn't the ASUS router routing packets from the 192.168.2.x subnet to the 192.168.50.x subnet, only the other way around. Isn't the purpose of a router to route packets in BOTH directions between two different subnets?

Any insight from you is much appreciated.

172.97.239.186 (talk) 17:40, 6 January 2022 (UTC)

It looks like a protection from an external attack because the ASUS router apparently assumes that it is connected to the provider's network, not to your primary router. Ruslik_Zero 20:18, 6 January 2022 (UTC)

Thanks so much for your answer. Do you know of any way to disable this protection on the ASUS router so that it can freely route packets in both directions on the two subnets? 172.97.239.186 (talk) 23:04, 6 January 2022 (UTC)

It's hard to be sure of what your problem is, but I can take a guess.

When a device on your 2.x subnet sends a packet to your 50.x subnet, your DSL router needs to know where to route that package. They are often set up to forward all "unknown" addresses out to the internet, and so your package disappears out the wrong port (where it just gets thrown away as it's addressed to a reserved address).

This would explain why it works the other way round: when you send a package from your 50.x network to a 2.x address, your ASUS doesn't know where to send it so it sends it to the "uplink" port. There is the DSL router which does know where to send it.

You need to set up your DSL router to send these packages to 192.168.2.111, and you need to set up your ASUS router to accept these packages. How exactly to do this is device dependent, and can be complicated. Both your devices are probably built for consumer use and either hide or don't have advanced networking options. You might be able to install an alternative firmware like OpenWRT on one of your devices (it does at least have all the options you need), but I recommend that as a last resort only unless you want it for another reason. El sjaako (talk) 15:11, 7 January 2022 (UTC)

Thank you so much El sjaako for your excellent explanation! 172.97.239.186 (talk) 15:44, 12 January 2022 (UTC)

I had a similar situation in the past. What I did was turn off routing, and turn the wifi providing box into a switch. It was an option. Also just let the DSL connected device do the DHCP and have it turned off on the WIFI. Also, just to get physical ports you can purchase an additional switch. Graeme Bartlett (talk) 20:15, 10 January 2022 (UTC)

aha! Let me try the switch mode! Thank you so much for this invaluable advice! 172.97.239.186 (talk) 15:44, 12 January 2022 (UTC)

Resolved