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Re: network setup question

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:

- 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.

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.

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