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Combining two subnets

Posted on 2008-10-31
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Last Modified: 2012-06-27
I have two locations. One is a 192.168.1.0 network and one is a 192.168.2.0 network. I am putting a line inbetween the two networks and I want them to be able to talk. I am not routing, just a simple switch.

I was under the impression that all i need to do is change the subnet mask in both ranges to 255.255.253.0. Is this correct?
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Question by:imagnl
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12 Comments
 
LVL 43

Expert Comment

by:JFrederick29
ID: 22852293
Actually, the mask would need to be 255.255.252.0 at a minimum.  Is the line between locations an ethernet line?  If so, this should work.  If a T1, there is more to it.
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Author Comment

by:imagnl
ID: 22852443
it is a Metro-E line,
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Expert Comment

by:JFrederick29
ID: 22852477
Okay, should work then.  You just need to change the subnet mask on every device.
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LVL 10

Expert Comment

by:MPCP-Brian
ID: 22852535
Assuming there are no routers involved, as you stated, make sure each computer has the same subnet mask and you can use either of the options below:

Option1: Subnet of 255.255.252.0 and this will give you a complete range of 192.168.0.1 through 192.168.3.254.

Option 2: Subnet of 255.255.254.0 and this will give you a complete range of 192.168.0.1 through 192.168.1.254 ... OR ... 192.168.2.1 through 192.168.3.254.

Since you span 192.168.1.xxx and 192.168.2.xxx, you can use option 1 or change one of the network subnets, depending on security.
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Author Comment

by:imagnl
ID: 22852921
I dont want to include 3 though. Can I use 253 if I just want 0, 1 and 2?
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Expert Comment

by:JFrederick29
ID: 22852969
Nope, 255.255.253.0 is not a valid mask.  It isn't a big deal if you aren't using 3.  In fact, it is there for future use if you need it.
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Author Comment

by:imagnl
ID: 22853056
It is a big deal because I have another facility that we route to that is .3
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LVL 43

Expert Comment

by:JFrederick29
ID: 22853088
Okay, you didn't mention that.

Well, it may be simplest then to route between metro-e locations.  Otherwise, you will need to address one side and use a 255.255.254.0 mask.  192.168.0.0 and 192.168.1.0.
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Author Comment

by:imagnl
ID: 22853115
What about going with 255.255.0.0?
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Expert Comment

by:JFrederick29
ID: 22853129
That just makes it bigger. 255.255.254.0 is the only one you can use unfortunately or keep two seperate 255.255.255.0's and route.

255.255.0.0 is 192.168.0.0 - 192.168.255.254
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Expert Comment

by:MPCP-Brian
ID: 22853198
Since you don't want 192.168.3.xxx, you're only option is subnet mask 255.255.254.0 and change your 192.168.2.xxx subnet to 192.168.0.xxx, leaving the 192.168.1.xxx alone (option 2 in my original post).
Good luck!
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Accepted Solution

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Mysidia earned 125 total points
ID: 22858504
Is that  192.168.3.0   a big network?  Do the other networks need to communicate with it often?
I see 5 options:
* Either stick with routing  between all networks
* Just change the subnet mask on the two networks anyways to 255.255.252.0.   Setup a router to utilize proxy arp on behalf of the third network.   This is suitable if the third network is small.
* Renumber the third network, so you can change the subnet mask of two other networks
* Renumber the other two networks, so you can change the subnet mask and combine just them
* Combine ALL 3 networks instead of just 2.



If you want to combine the two networks to the same Layer 2 network,  then you need to renumber the third so it will not be included in the subnet mask,   or mesh the third network as well by getting  Layer 2  bridges setup between the third network and each of the other two networks.

Should probably be two Bridges from the third network for redundancy,  each to the other two, with spanning tree protocol to help protect against loops, and the third network would need the same netmask change. I do not generally recommend expansive bridging like this this:  bridging across WAN interfaces is a bad idea, unless your WAN connections are comparable to your LAN connections.


Depending on the usage of that third network compared to the other two, it may be possible to NAT the 192.168.3  network using a static translation  as a transition mechanism.


It is generally ideal to place private networks on different /22s  or even different /20s, if you will not be ever combining them.
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