NAT router confusion

Ulf Magnusson ulfma629 at
Fri Jun 24 10:25:53 GMT 2005

----- Original Message -----
From: "Michael H. Semcheski" <lists at>
Date: Friday, June 24, 2005 1:46 am
Subject: Re: NAT router confusion
> On Thursday 23 June 2005 07:43 pm, Ulf Magnusson wrote:
> > Is this router really some switch/router hybrid? Or..? Bleh, someone
> > please sort this out for me. I realize this isn't strictly
> > FreeBSD-related, but I simply couldn't think of a better place to 
> pick> brains, so I hope I'll be excused :)
> It is a switch / router hybrid.  If the traffic is going to an 
> address on the 
> same network, its a switch.  If the traffic is going to an address 
> on a 
> different network, its a router.
> If you understand that concept, then you should have a pretty good 
> idea of how 
> the system works.
> I do not have a complete enough understanding of IP networks to 
> explain this 
> in specific detail.  I think the key is that the computer 
> generating the 
> traffic looks at the netmask for the sending interface (eg, 
> and uses this to determine if the endpoint of the traffic is on the 
> same 
> network or not.  If it is, it sends the traffic directly to the 
> host.  If it 
> is on a different network, it forwards the traffic to the gateway 
> address.
> Mike

Thanks, I think I understand how it works now. I guess it's basically
like an ordinary router that pretends it's a switch for all addresses
that appear on the same local network. It looks at the destination
address in IP packets and the address of the sending system and goes
into switch mode if they both appear on the same subnet (which is pretty
much verbatim what you said, when I think about it).

I'll throw another short question in the mix while I'm at it.. perhaps I
should rename the thread "Switching/routing questions from a curious
networking newbie" :-)
Do switches gain anything by having full-duplex connections to hubs? I
understand there must be a performance benefit when you connect a host
directly to a switch, but won't the half-duplex connections of the hosts
to the hub become a bottleneck?


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