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Changing entire network's IP addresses

Peter Doyle
Occasional Visitor

Changing entire network's IP addresses

Hi guys,

I need to change our network's IP addressing from 192.168.1.xx/24 to something new i.e. 192.168.24.xx/24. I was going to do it all at once since our network is pretty small, but we just installed some Procurve 2650's and I'm curious if there's a better way to do it, with less downtime.

Anybody been through this before? The problem is that most things are on DHCP (workstations), but some are on static IPs (servers, printers, etc), so it'd be nice to have the two networks talk to each other for a while while I change the static ones.

Is it possible to have the two networks talk to each other during the transition? Also, what about DHCP? Can I make the DHCP server on the 192.168.1.x network place clients into the 192.168.24.x network? (using DHCP on Win2k3).

Sorry if this has been covered before. I wasn't quite sure what to search for.
Pete Doyle
Paul Clayton_1
Frequent Advisor

Re: Changing entire network's IP addresses


Our company went through this exercise 18 months ago. What we did was identifiy all the potential pitfalls, equipment and what was needed to do. Our requirement was to change ip addresses of 250 users, 8 switches, 7 vlans and 28 servers. It took us 15 hours to accomplish.
With good planning, you can accomplish this quickly and easily. The trick we found was to have 100% working DNS, and prepolulate the DNS prior to the change.
Sergej Gurenko
Trusted Contributor

Re: Changing entire network's IP addresses

1. Add a secondary address to all important servers (as well internal DNS). Change DHCP scope subnet, and let the Clients PC's to learn new addresses (new default GW, new DNS, etc). Wait for the moment all PCs get a new address and remove primary IP from servers.

If you do not use Wins and very old NT systems you will not have a problems.

2. I'm not sure what devise is the default gateway. If it is not a Firewall (usually they can't route traffic back to the same interface) You can add one more subnet to the Default Gateway interface. You will have the connectivity between all new and all IP addresses during migration.
Preston Gallwas
Valued Contributor

Re: Changing entire network's IP addresses

Oh yeah? We've got 45 sites with 65 subnets, using a 152.157.x.x / 24 implementation (internally) that we need to renumber to 10.x.x.x, or 172.16.x.x, or 192.168.x.x


Ive got a pretty thoroughly documented plan worked out, but management hasn't signed off on it due to the labor involved with modifying all those DHCP servers and such, but heres the jist of it:

Set up sub-interfaces on all the routers with the new network scheme. Set up new routes for subifs (in our case, 10.x.x.x /16 as a master summary of all of our routes in OSPF)
-Test a 10.x.x.x numbered device on each network (ensuring it can talk to everything)
Configure the 45 DHCP servers to hand out only 10.x.x.x addresses with the new default gateways
Renumber printers
Reinstall printers on user machines (heres a 6 month project)
renumber servers - modify DNS to match
Re-do DNS other static DNS entries (we use Dynamic DNS)
Change the subif to primary if
remove subif

Peter Doyle
Occasional Visitor

Re: Changing entire network's IP addresses

Thank you guys for the responses, they've been really helpful.

I will try to work up a more detailed plan in the next couple days, but the main things I think I need to do are:
- make sure all devices (computers, printers, etc) are set to grab IPs from DHCP.
- make sure everyone's printers are set to grab IP's from DNS (instead of being hard-coded IP addresses).
- add secondary IP's to the main servers
- Do whatever it takes to get the two subnets to talk together (this is where I still need help - see below)
- Put a computer onto the new subnet and test to make sure I can still talk to servers, printers, etc
- Make all port-forwarding on our firewall point to the secondary/new IPs on our servers.
- Test to make sure all external services (email) are still working after changing the port-forwards
- Set DHCP to give the new network's numbers.
- Force all endpoints to grab new IP (reboot or ipconfig /renew, etc)
- Test to make sure everything is still working.
- Once satisfied, remove old IP addresses from servers.

Hopefully I'm on the right track there. We really only have ~20 workstations and a couple servers, so its not a huge deal. I just want to keep email up during the process.

My biggest question now is how can I get the two subnets to seamlessly talk to each other during the transition period?

Everything is currently set to have our firewall as the default gateway. It definately has the ability to route between them, but I think the ProCurve 2650 would have better performance if I can get it to do the routing.

To do that, would I have to set the main 2650 as the default gateway on all the computers?

Once I do that, what is the best way to configure it to allow subnet 1 to talk to subnet 2?

I think I just need to set up two VLANs and have the 2650 route between the subnets. I think that will stop all broadcast traffic (?) but we don't use that much. Just DHCP, I think.

I also just saw something about multi-netting, but haven't had a chance to research it yet. Maybe that may help with allowing broadcast traffic?

Thanks again for all your replies, I really appreciate it