[CentOS] Using two subnets to change network configuration

Thu Jul 26 01:07:05 UTC 2012
Cliff Pratt <enkiduonthenet at gmail.com>

On Thu, Jul 26, 2012 at 12:52 PM, Harold Pritchett <harold at uga.edu> wrote:
> Problem:  My network uses the network.  Since is the most
> common network in all of the world it begins presenting problems when I want
> to set up vpns, or try to do other routing.
> The solution:  Change the network from to  This
> is somewhere in the middle of the less frequently used RFC-1918 20 bit private
> network range.  My network contains 40 or so nodes, windows, linux, and
> proprietary operating systems of various types (TiVos, iPhones, iPads, TVs,
> BluRay players, WAP's, etc.)
> What I would like to do to minimize down time would be to create a single new
> machine on the network with addresses on both networks and set up as a router
> between the networks.
> That way, I can convert the machines one at a time, and not loose any connectivity.
> When all done, take the temporary router down and just use the new addresses.
> Machines are set up both static and DHCP, and there are some virtual servers
> running on a VMware machine.  Updating all of the machines using DHCP can
> be done in a single pass, but the static machines will have to be done one at a time.
> Can anyone give me any pointers to web sites to help set this up. I know how to
> set up multiple addresses on the same NIC (eth0 and eth0:0).   After that, I'm at
> a bit of a loss.
I can confirm that this works - I've done it a few millennia ago. As I
very vaguely recall you need to turn on forwarding and set up routing
on the router machine. Also any routers on the network segment might
need to be changed to send traffic destined for the new network to the
router. You might have issues with DNS, WINS, etc. You would need to
change any routers to not use the temporary router when you switch to
the new network.
> And Yes, I know setting up multiple subnets on the same physical wire is a bad
> idea, but this is only for a couple of days until I can get everything moved to the
> new address scheme.
Why? It can be confusing, yes, but many people route VOIP over the
same cabling to desk phones.