VM Router in Modem Mode + External IP

[Azzaka]

Question, 

I placed the VM Router into Modem mode (VM Modem from now on) and use my Own router on the Ethe connection garnishing the External IP, then doing what it should be doing and sending an Internal IP, internally. All good.

An engineer visited today and when he connected to the VM Modem whilst my router was connected, he also gained the same external IP as the router. How is this possible? both connections were working as intended.

In short, this broke my noodle.

Regards,

[Roger_Gooner]

There is a bug in some hubs whereby in modem mode more than one LAN port can be active at the same time.

The reason for getting the same IP address is that the hub, or its modem to be a bit more exact, stores the WAN IP address following communication with the CMTS, and it is this address which is passed to the router. Even if the hub were rebooted and another IP address obtained the probability is that it will be the same as before as they tend to be quite sticky even though dynamically assigned.

[legacy1]

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@Roger_Gooner wrote:

The reason for getting the same IP address is that the hub, or its modem to be a bit more exact, stores the WAN IP address following communication with the CMTS, and it is this address which is passed to the router. 

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Surely thats not ture? unless newer hubs do that? but the way I know it works is the hub knows nothing about what IP you get in modem mode (well 192.168.100.1  does but thats because you made the connection to it for the reply back) in that DHCP goes in the hub and out to the server then reply comes back from server and out the ethernet port.  

[Roger_Gooner]

Of course the hub knows about the WAN IP address because the modem is part of the hub.

[legacy1]

Thats true for router mode but thats not how modem mode should work.

But going back to Azzaka with the hub in modem mode the WAN IP on their router and engineer getting the same WAN IP suggest some type of DHCP/ARP proxy and its unlikely both use the same MAC so two different MAC's get the same WAN IP and even ARP allows both to see the gateway on different MAC's. Not saying its imposable but seems a lot of work for VM to pull such a thing off without problems.

In fact the tech looks like what I posted years ago...didn't think anyone was taking me seriously to really do something like it

https://bridgemode.bounceme.net/modem%20bridge%20mode.png

also another blueprint https://bridgemode.bounceme.net/nat102.png

[Azzaka]

I can personally say that the 192.168.100.1 does not always work for me since they upgraded the kit in the exchange about a month ago. I need to use 192.168.0.1 for the VM router in Modem mode and 192.168.1.1 for my router.

Still not sure how the external IP is shared even though I follow the above comment. In 'Modem' mode my understanding is that it acts like a 'passthrough' however it appears it is still acting like a router and assigning the external IP to all ports on the back. This is NOT the way a modem works as far as I was taught. I could be wrong.

Also, thank you @Roger_Gooney for your reply.

[Tudor]

On the newer hubs you can always get to the hub in modem mode on 192.168.0.1 IF you own network subnet is not 192.168.0.0/24