Routing Between Subnets

[Piyono]

We're switching from DSL to Cable.

Until I wire everything in I plugged a Mercku mesh router directly into the cable modem, as you can see in the diagram.
The modem's LAN IP is fixed at 192.168.100.1 (even though we purchased the modem we don't get management access ).
The Mercku router is serving DHCP in the range of 192.168.127.x.

It would seem to me that a computer receiving an IP address via the mesh would not be able to access the cable modem without a route explicitly established in the router. Despite my intuition, I am able to browse to 192.168.100.1 from any computer on 192.168.127.x.
I've been through every page of the router's firmware but I don't see any routes back to the modem.
What could explain this behavior?

 

[Chewy509]

There must be something on the 192.168.100.x connected to the mesh and forwarding... Are you sure the Mercku router isn't connected the WLAN setup? Are you sure your devices are not connecting to both mesh and WLAN setups at the same time?

Try running a trace route from source to destination? should give you the way it's crossing the subnets, also have a look at the arp tables of both nodes after running the trace route...

IIRC,
Windows command line = 'tracert'
Linux command line= 'traceroute'

 

[Newtun]

In Linux, you might have to install traceroute; from my "vanilla" 16.04 Ubuntu install, that looks pretty easy:

ubuPower:~$ traceroute storageforum.net
The program 'traceroute' can be found in the following packages:
* inetutils-traceroute
* traceroute
Try: sudo apt install <selected package>

 

[Piyono]

Aside from one old iMac (and some recreational VM-ing on my part) this is a Windows 10 household.

Tracert shows a single hop to modem; predictably, through the Mercku (for which my dislike is increasing what with its instability and snowflake temperament).

The modem doesn't show up in the ARP tables.

I've emailed Mercku support. Still waiting for a reply.

 

[Piyono]

Mercku Replied:

Could you please elaborate more on what you would like us to assist with? The Merckcu router is in the modern subnet, it is normal for devices connected to Mercku to access modern. So we don't quite understand your problem. Could you please explain more to us?

Is that right? Wouldn't I still need to manually give a computer a 192.168.100.x IP in the correct subnet in order to access the modem?

 

[Chewy509]

You would be correct, a 192.168.100.0/24 and 192.168.127.0/24 shouldn't be able to talk to each other, without explicit gateway or routing rules setup...

Can you recheck the subnet setup on all components, as the default for 192.168.x.x is a /24 (or 255.255.255.0) subnet. Is it possible the Mercku is on a /16 (or 255.255.0.0) subnet?

Alternatively, can you change the Mercku to a 10.x.x.x address with a /24 subnet, and see what happens?

That way, the modem's network is 192.168.100.x and the Mercku network is 10.0.0.x.... Hopefully that'll make it easier to diagnosis and explain to their support...

 

[sdbardwick]

If the Ethernet cable from the DOCSIS modem is plugged into the WAN port on the Queen router, then behavior is as expected for any devices connected to the mesh, is it not?

 

[Piyono]

Hmmm... It could be a /16.
The sticker on the back doesn't specify. I don't have management access to the modem and the guest overview doesn't provide that information.
Is there any other way of finding out?

 

[Newtun]

When you say you "don't have management access to the modem", do you mean you can't connect to the web interface at all, or that there's a login screen but you don't have the ID and password? Maybe it's something simple like "admin"/"admin", or you can Google and find the default ID/pwd.

 

[Piyono]

When you say you "don't have management access to the modem", do you mean you can't connect to the web interface at all, or that there's a login screen but you don't have the ID and password? Maybe it's something simple like "admin"/"admin", or you can Google and find the default ID/pwd.

I mean that password access to the modem has (apparently) been disabled by (presumably) Rogers Cable, and thus the management console is unavailable. Accessing the modem at its LAN IP address presents a password-less guest overview page of little utility.

I've been hounding my ISP for admin access but they're giving me the runaround saying they don't have access, themselves. I'm not a direct customer of Rogers Cable. I'll pester them as well but experience suggests they'll say "talk to your ISP".

 

[Chewy509]

I want to clarify the setup for a moment:

1. You currently have 2x internet enable services, one via cable and one via ADSL.
2. According to it's datasheet the cable modem (Hitron CDA3-35) is a modem, with builtin router.
   1. It's default IP address is 192.168.100.1.
   2. The default DHCP server will provide a range of 192.168.100.0/24
3. You have a Mercku AP providing wireless mesh services, and you have enabled it's DHCP server to provide 192.168.127.0/24 addresses.
4. The Mercku is connected to the cable modem via an CAT5/6 cable?
5. You have a TP Link Archer C7 router connected to a TP Link 8616 ADSL modem. The Archer C7 is providing a gateway service via NAT and I assume is setup to provide DHCP? What is the DHCP range for the Archer C7 providing?
6. You currently have PCs/Tablets on the LAN/WLAN serviced by the Archer C7. All these are accessing the internet via the ADSL service. (The gateway IP they are using is the Archer C7's IP address).
7. However, you have an issue where PCs/Tablets on the LAN serviced by the Aracher C7 are able to reach the Cable modem's interface on 192.168.100.1, and would appear to be doing this via the WLAN Mesh setup of the Mercku ?
8. Are any of the PCs / Tablets configured to use the Mercku mesh WLAN?
9. Has the Mercku router been configured to connect to the WLAN setup provided by the Archer C7 or DLink DIR-601 AP?

Is this correct?

What I think is happening is:

1. You have the Cable Modem, on 192.168.100.1/24.
2. You have the Mercku connected to the Cable Modem, and it's getting an address of 192.168.100.2/24
3. You have the Mercku providing an additional 192.168.127.0/24 subnet via it's WLAN mesh service.
4. Anything connected to the Mercku mesh network will be able to reach the 192.168.100.0/24 subnet, as the Mercku routing table knows where the 192.168.100.0/24 LAN is.
5. You have configured your PC's/Tablets to connect to the Mercku mesh WLAN and the Archer C7 LAN at the same time...
6. Therefore, what you are seeing is correct behaviour...

A few points:

1. Only have 1 DHCP service on the LAN, and if you can't get access to the cable modems configuration, this will have to be it.
2. If you turn off the Mercku, can PCs/Tablets on the Archer C7 LAN connect to the cable modem interface?

Source:
https://mediacomcc.custhelp.com/euf/assets/documents/modem user guides/Hitron CDA3-35 User Guide.pdf
See pg 39 for resetting the unit, and the default username/password for admin.

 

[Piyono]

Clarification:
1) Yes, but cable is replacing ADSL, which should be gone by month's end.
2.0) Yes
2.1) Yes
2.2) I don't know what the subnet mask is.
3) Yes. DHCP range is 192.168.127.100–199.

4) Yes.
5) Yes. DHCP range is 192.168.1.11–31 (although once DSL goes away the Archer C7 will be repurposed).

6) Correct
7) No, only PCs on the Mercku router can see the modem.
8) Everything is in flux. The Mercku has been unreliable so everyone's hopping back and forth between networks as necessary.
9) No, it's completely separate.

There's no local network per se; just individual devices connecting to the internet. Each device is connecting to only one wireless network at a time. There's definitely no DHCP mixup happening.

I realize that the Mercku router is plugged directly into the modem therefore knows how to route to it but I assumed as follows:
1. Everything is configured as /24
2 .That being the case, 192.168.100.x should not be able to talk directly to 192.168.127.x unless explicitly routed.

 

[Chewy509]

I think you've answered your own question, the Mercku router is routing between subnets for you...

 

[Piyono]

Yes, that seems to be the case.