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  1. DSL modem with 4 port LAN router. PPPoE config to internet. LAN IP = 192.168.0.1 DHCP server enabled. 192.168.0.200 - 254 served
  2. 4 port Linksys wi-fi LAN router, connected to Modem, also DHCP enabled for itself (gets IP from modem) and as a server on a different subnet. 192.168.1.101 - 201 served.
  3. Multiple PC's connected to router and to modem.
  4. Subnet mask for EVERYTHING as far as I can tell is 255.255.255.0

I want to know why/how everything on my network can talk to everything else, even if configured on different subnets. And all pcs are able to access the internet.

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    what are you trying to accomplish? the linksys is managing the 192.168.1.0/24 network. being plugged into the DSL modem, gives the DSL modem a route to the 192.168.1.0/24 network. Absent firewall rules denying traffic, requests to any hosts connected to the linksys from a host on the DSL modem is possible, and vice versa. – Timmy Browne Jan 30 at 17:39
  • The cable from the DSL modem/router to the Linksys router -- does it go to the Linksys router's WAN port, or to one of its LAN ports? Is the Linksys router configured to do NAT between its WAN port and its LAN ports? – Spiff Jan 30 at 17:44
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    @TimmyBrowne No, just plugging the Linksys router into the DSL modem/router doesn't give the DSL modem/router a route (a routing table entry) to the remote subnet. You would have to use a route advertisement protocol to do that, and those are usually not enabled by default. Otherwise you'd have to configure a static route in the DSL modem/router so it knows that the other subnet exists, and that the Linksys box is the gateway. – Spiff Jan 30 at 17:46
  • The only reason I'm going down this road of questioning is that in the past, I have never had luck crossing traffic between subnets. I have always thought DHCP everywhere would be the easiest, but with older devices in the network that have limited configurability (stuck on the 1.x or 0.x sub) visibility from one to the other has always resulted in 404's or similar. I wasn't actually trying to make this work, but it did through blind luck and I don't understand why it is different this time. – ultrageek Jan 30 at 17:48
  • @Spiff - the connection from the router to the modem is through the router "internet" port, not a lan port. – ultrageek Jan 30 at 17:50
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A host on 0.1 doesn't know how to get to 1.0 because 0.1 doesn't have a route to it. 1.0, on the other hand, has a default route to 0.1 so that's how it gets to 0.1 and the internet. All unknown addresses get sent to their default route and if that doesn't know about it, it then sends the traffic to it's default route. A host on 1.0 sends traffic to it's default route which happens to be 0.1 so it gets routed properly but the default route of 0.1 is the internet so it can't get to 1.0. If you can add a route on the modem to 1.0, then 0.1 can ping 1.0.

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There are a few possibilities depending on the config

  1. Devices behind the ethernet router are being double nat'd - in this case they can access devices behind the DSL modem but devices directly connected to the DSL modem cant initiate connections to devicesbehindn the ethernet modem. From what you describe this is actually the most likely scenario.

  2. It is possible that NAT has been disabled on the ethernet router and the DSL modem has a route back to the ethernet router for 192.168.1.0/24 - the ethernet router can just rely on its default gateway to pass traffic off to the DSL router which in turn knows how to route to 192.168.0.0/24

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