Home Network Setup Problem

deltaski at earthlink.net deltaski at earthlink.net
Fri Sep 9 12:58:02 PDT 2005


On Friday 09 September 2005 02:14 pm, Greg Barniskis wrote:
> deltaski at earthlink.net wrote:
> >>Is it a switch, is it a router, or is it really both (high end
> >>thingy like Cisco 35xx?). Probably it is just a plain old switch
> >>with no routing capabilities. To avoid confusion, you should call it
> >>what it is.
> >
> > Oh my, sorry. It is an 8-port 10/100Mbps Ethernet Switch! How does that
> > change anything?
>
> It really doesn't (you don't want a router in that location, you
> want a switch). A router connects multiple IP subnets that otherwise
> cannot talk to one another. Turning on the gateway feature on your
> FreeBSD box makes it a two-interface router. A switch merely
> multiplexes packets on many ports (it's a signal repeater/amplifier).
>
> [snip]
>
> > Oh, my sorry!  Yes, the default gateway is set and I have no firewall to
> > complicate matters.
>
> Ah... I see the problem now. You *MUST* do NAT on your BSD gateway,
> unless you personally control the configuration of your DSL router
> and can give it the necessary routing instructions to find your 172
> network.
>
> You are trying to ping your DSL router from a private network
> address that the router does not know about. The ping will reach the
> DSL router and it will not know where to send the reply because your
> private address does not (cannot) exist in its routing table. So, it
> sends the reply on its default route, which is towards the Internet.
> Bye, bye ping reply!
>
> Again, this is just very basic networking stuff. I didn't see it
> before because I route packets between private networks all the time
> and it works -- the difference is that all my routers are
> well-informed about the pathways to all nearby networks.
>
> For the background information you need to know, buy this or find it
> at your local library:  http://www.oreilly.com/catalog/tcp3/
>
> I'm sure there are other and even better titles.
Greg, thank you for the link.
            But, I do have one problem with your analysis. If the DSL router 
(192.168.1.1) received a ping from 192.168.1.100 would it not respond to 
192.168.1.100 as that is a known address? If 192.168.1.100 can connect to the 
internet thru 192.168.1.1 it is indeed reachable from the DSL router 
192.168.1.1! As such, the problem would be no packets getting to 
192.168.1.100 from the other available network (172.16.1.xxx). Remember, NIC 
192.168.1.100 can ping and connect to the internet. 172.16.1.35 can ping 
192.168.1.100 BUT, 172.16.1.35 CANNOT ping 192.168.1.1 thru 192.168.1.100!

Donald


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