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I have the following..

eth3 - default route, usb dongle to the internet. 192.168.8.0/24    
eth2 - nic to wireless router for streaming webcam 192.168.3.0/24    
eth1 - nic to wireless router for phones/computers 192.168.2.0/24    
eth0 - Wired for well wired stuff... 192.168.1.0/24

All of these things can talk to the internet if they wish, all have DHCP running, and generally work.

However I cannot make eth0, 1 or 2 talk to each other. I can't ping 192.168.3.2 from 192.168.1.10, it should head from 10 to the server, get routed to the next ethernet and handpassed on, but it stops dead.

I have forwarding enabled. All eth0, 1, 2 are masq to eth3.

Anyone have any clues as to what I might be missing?

2 Answers 2

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It's because they are on different networks. 192.168.0.0 is private addressing. Outside of its own network, private addresses are ignored. /24 identifies how much of the address is allocated as a network identifier, in this case 24 bits, or three 8-bit sets worth. Those networks are 192.168.8, 192.168.3, 192.168.2, and 192.168.1; all different private networks, which means they are all mutually ignoring each other. They need to be on the same network to talk, so they either need to all be on 192.168.1.x (or 2 or 3 ...) /24 network OR all need to be on the 192.168.x.x /16 network.

The reason you can talk to the internet, by comparison, is because you don't talk to the internet under a 192.168 address. Somewhere, probably through a router, your address is being converted to a legitimate global address.

EDIT: Corrected language which was technically incorrect. 192.168 are allocated as private networks. Link local are 169.254 addresses. Fundamental problem is the same, though.

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  • really? I used to have this working somehow... mind you I certainly can't replicate it now. I was hoping not to have to subnet it, that's a pain in my patooty. I would've thought I should be able to route these between 3 private nets though. Jul 20, 2018 at 1:28
  • You can, but... the problem is they can't talk directly, because... ignored. You'd need an intermediary device, like a router, that converts it to an address that the receiving network can talk to, like your router does with the internet.
    – Chris R.
    Jul 20, 2018 at 1:38
  • like 192.168.1.2 -> 192.168.1.1/192.168.2.1 -> 192.168.2.2
    – Chris R.
    Jul 20, 2018 at 1:40
  • that's what the ubuntu box is meant to be doing... Jul 20, 2018 at 1:44
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Hold the phone, my bad! I should've thought of this much eariler. It is in fact the wireless router at 192.168.3.2, that won't answer to 192.168.1.x., which will be a security thing. Other hosts out on that network are in fact working and answering just fine, I should've put some other devices out there to check. Sorry to waste peoples time.

To think I spent all night trying to fix something that wasn't fundamentally broken :(

Aunty

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