Home Network Setup Problem

Greg Barniskis nalists at scls.lib.wi.us
Fri Sep 9 12:18:10 PDT 2005


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 Barniskis, Computer Systems Integrator
South Central Library System (SCLS)
Library Interchange Network (LINK)
<gregb at scls.lib.wi.us>, (608) 266-6348


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