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 192.168.1.1, host A with 192.168.1.2, and
> host B with 192.168.1.3. All have a subnet mask of 255.255.255.0.
> 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
> 192.168.1.3, then it will first send out an ARP request to get the
> Ethernet address of 192.168.1.3. 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
> >> 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 192.168.11.0/24
(for my example I will use 24 instead of 27)
the server would have an address 192.168.11.55/24 (for example) and the
router would have 192.168.11.1/24
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 192.168.11.55/22 can't
talk to 192.168.11.1/24 (you can fake it on linux with iproute - see my
other answer to this thread).