LinuxQuestions.org - 2 subnets over the same cable

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2 subnets over the same cable

Hi everyone.

Here's my odd request.

I have a 2-floor house, with a single ethernet cable from the ground floor to the 1st.

Downstair there is my ADSL modem, which is connected to the router upstair through this one cable.

Now, I would like to have some more ethernet ports on the ground floor, but there's no room left to run another cable between the floors, so here's the big question: can I run both the modem-router connection and my regular LAN (which are on different subnets) through that one cable, with a switch at both ends?

To make it a little more clear, here is my present configuration

Code:

DOWNSTAIR          UPSTAIR

ADSL Modem -------  Router --- Clients

Here is what i would like to do:

Code:

DOWNSTAIR                    UPSTAIR

ADSL Modem ---- Switch ------ Switch ----(WAN Port) Router --- Clients        

  Clients ----                      ----(LAN Port)

Would it work?

You're not very clear on what these two different networks are, is it a small uplink subnet between the router and the modem, and then a client subnet behind the natting router? If so, why not just move the router downstairs next to the modem? As for the technicalities of what you're asking, given that you're basically just asking to run two ip subnets on a single untagged physical subnet, then yes it will work. It's an awful awful thing to do, but it'll work.

Thanks for your reply, Chris!

I'm not moving the router downstairs because it has built-in wireless, and i need it mostly on the first floor, so I'd have to get another access point to get a decent speed upstairs.
I understand that it's not a good thing to do, but it shouldn't be a problem as long as bandwidth is concerned: my connection is really bad, since I'm far far away from the exchange (about 6 km), so it's only a ridiculus 640/256 kbps, and I only need a few tens of megs from the LAN.

How big are your floors? My WAP is on the first floor of my house and I can still get 70%+ signal setting in my attic (yes, I've tested), and all the way out to the back street that I park along I can still get a strong connection.

they're not that big and i have a good signal all over the house, more than good enough to surf the internet, but not enough to get files from the nas, as the speed is less than 1 megabyte per second

How about go here, & build one of these:

Then you could move the router downstairs.

I have had a half-size #2 working for about a year at the end of our computer club's suite. It's set up to beam the signal back toward the center of our space.

Chris mentions "a single untagged physical subnet", I think he'd like to recommend 1 or more VLAN-ing switches or routers. Unfortunately, their price may be prohibitive. If you're interested, I could get some (comparatively) low-cost recommendations from a friend that has several installed at his work. If you can afford it, this is probably the most robust & elegant solution.

Finally, a (horrible?) h/w kluge occurs to me: build custom adapters for each end of the ethernet cable that split the 2 unused pairs off into a 2nd ethernet circuit. (Assuming it's 10/100.)

BTW, it's normally bad form here to question the OP's constraints on a solution; but how bad is the problem of stringing another ethernet cable? -- I do some cabling & some carpentry, so I am curious.

With a pair of antennas like the 2nd you mentioned I managed to get stable a 2,5+ km link 2 years ago :)

Anyway I set up what I described on my 1st post and it seems to work quite reliably, for my needs it's sufficient.

BTW, upstairs everything is gigabit, while downstairs it's 100 Mbit, so the custom adaptor solution would work, but it would be too much trouble to build them...