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Re: network setup question

On Wed, 2010-05-05 at 22:25 -0700, Kevin Ross wrote: 
> On 5/5/2010 9:11 PM, Alex Samad wrote:
> > On Wed, 2010-05-05 at 20:55 -0700, Kevin Ross wrote:
> >    
> >> On 5/5/2010 6:06 PM, Miles Fidelman wrote:
> >>      
> > [snip]
> >
> >    
> >>> - I have two network /27 network blocks that are NOT contiguous - I
> >>> use one for each box
> >>>
> >>>        
> >> I'm not a networking expert, but this part seems wrong to me.  I don't
> >> think you're supposed to have different subnet addresses on the same
> >> broadcast domain.  If they both had the same subnet address, they would
> >> then talk to each other over the switch and not touch the router.
> >>      
> > Why do you think this, reason I ask is I had a rather long discussion
> > with a work college about this and I am wondering were this thinking
> > comes from.
> >    
> Which part?  The part about different subnets on the same switch or 
> hub?  If so, yes I guess there's nothing terribly wrong with doing that, 
> other than causing extra traffic to the router between subnets when they 
> could be talking directly to each other.

The question was why did he think it was bad/wrong to put to different
subnets on the same broadcast domain.

the extra taffic would be arp requests and broadcasts, but with all
unicast packets a switch will switch ie 1 port to another port.

There is no technical reason that ip subnets have to be on seperate
broadcast domains - there might be security and other reasons.

> Or is it the part about the two computers talking directly to each other 
> without the router if they are on the same subnet?  If so, I'm speaking 
> strictly of TCP/IP over Ethernet.  Let's say you have a simple network 
> with a router IP address of, host A with, and 
> host B with  All have a subnet mask of
> Since we are operating over Ethernet, the TCP/IP stack needs to 
> determine the Ethernet address to which to send a packet.  If host A 
> wants to send a packet to a host within the subnet, for example to 
>, then it will first send out an ARP request to get the 
> Ethernet address of  When it receives it, it will then send 
> the packet over Ethernet to the address received via ARP, which will 
> cause the packet to go straight to the receiving computer, not to the 
> router (if using a switch not a hub).
> If the destination IP address is outside of the subnet, then it asks ARP 
> for the address of the router, and sends the packet over Ethernet to the 
> router.
> >> Another option is to change the subnet mask so that the mask then allows
> >>      
> > careful you might loose connectivity  with the router.
> >
> >    
> I was only mentioning it as another possibility.  I don't think you will 
> lose connectivity with the router, just with other hosts that the 
> computer now thinks are in the same subnet, but really need to go 
> through the router.  But if you know that you'll never want to talk to 
> those hosts, then this is a viable option.

well think about it, if we are talking about network
(for my example I will use 24 instead of 27)

the server would have an address (for example) and the
router would have

if I change the netmask of the server it can no longer talk to the
router because it is in a different ip network ie can't
talk to (you can fake it on linux with iproute - see my
other answer to this thread).


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