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How do I figure out what subnet my modem is on without guessing at it? (alien.top)

submitted 1 year ago by [email protected] to c/[email protected]

7 comments fedilink hide all child comments

I got my ISP to put my modem into bridge mode, so it acts as ONLY a modem. I have my own router, which is on 192.168.0.x with DHCP on and some static IP assignments. Everything works fine. But my modem is on a different subnet, and honestly it changes. It WAS 192.168.40.x but I THINK now it's 192.168.100.x - I have no control over it, it's my ISP that changes it and I have no idea why.

What I want to do is to be able to access my modem management interface so I can see if it's connected to the internet (for troubleshooting, when my internet fails - is it my modem? router? etc.) I can't do that from a PC on the 192.168.0.x subnet obviously. But it's really annoying to physically connect a PC to the modem and set static IPs GUESSING at what subnet the modem management portal is on.

Any advice? Ideally, I'd love for a way to access the management portal from any PC anytime... I guess the solution there would be to put everything on the same subnet? More difficult than it seems when ISP decides to change it on a whim.

As an aside, I tried setting a static IP on a laptop hard-wired into the modem just now and neither 192.16.100.1 nor 192.168.40.1 were allowing me into the modem's management portal so now I'm at a loss. That said... this was while my modem was connected to my router still via a different LAN port. Do I have to disconnect all other LAN ports before it will work? Or has the modem's subnet changed AGAIN?

Thanks for your help.

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[–] [email protected] 1 points 1 year ago (4 children)

I'm not sure where the .40. address is coming from. My Arris modem's admin page lives at 192.168.100.1. I don't need to do anything to my router to access it.

The router knows that it doesn't own a 192.168.100.x subnet and forwards that traffic 'toward' its default router, where the modem will reply. Not all modems use the .100. subnet. Arris and Motorola do.

In a typical (Arris/Motorola) config, if you watch the ethernet traffic while the modem is coming up and the router is DHCP'ing for its WAN address, you will see the WAN get 192.168.100.xx address until the modem negotiates with the ISP. The modem will then drop and restore link to the router forcing the router to do another DHCP request. The response to this second DHCP request receives the public IP address for the router's WAN port.

If you run wireshark on a PC connected to the modem while powered off, then power on the modem, you should see a gratuitous ARP advertising the modem's IP and MAC addresses. This will probably be the management IP address of the modem.

[–] [email protected] 1 points 1 year ago (3 children)

This is good advice. Thank you.

It is 192.168.40.1. or it was. Google sagemcom fast 3896.

[–] [email protected] 1 points 1 year ago (2 children)

With a PC connected directly to the modem, can you access 192.168.40.1?

It does seem possible for the ISP to modify the admin IP address via the modem config file, but I have not seen it done. I also cannot come up with a way to find it (not enough coffee, yet), short of a packet capture with your PC directly attached to the modem and hoping the modem advertises its IP address once connected to the ISP.

[–] [email protected] 1 points 1 year ago (1 children)

Tried not 40.1 and 100.1 with 40.2 and 100.2 manually set IPs on a hardwired laptop. Neither worked.

Maybe some other user error? Should I not have set a default gateway while setting manual IP? Should I have disconnected modem from router before trying to access? Point is I wanna be able to access it to see what it's saying with less hassle. Doesn't seem like there's a silver bullet for me.

[–] [email protected] 1 points 1 year ago

Try setting your ethernet interface to get its IP address via DHCP. Let it get the default route from the modem, then see if you can access 192.168.40.1.

I took a quick look at what happens in my router when accessing my modem. It looks like the modem intercepts its own traffic. It should have been forwarded to my default router, but the default router shouldn't know anything about the modem. I'll get a packet capture when I get a chance.