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Planning a Network Wide Change of IP Addresses

It isn't unusual for a rearrange of IP address scheme to be required. The main reasons for this include:

• Either due to poor planning - using a registered IP address range that doesn't belong to you
• Changing from registered IP ranges to private ranges
• Rapid growth, meaning that you have outgrown the current scheme
• Conflicting with something else - for example if you use 192.168.0.x addresses then you will conflict with most home networking equipment - causing problems for remote access.

While changing the IP address range of a network sounds like a massive job, if you are a small business (less than 200 machines) then it is quite straight forward. Just plan ahead and do it in the right order.

Remember that as soon as you start changing anything it will start to break and fill the event logs up with errors. Apart from that Windows servers will cope quite happily with having their IP address changed.

Basic Plan

The plan we use for most clients is similar to this:

1. Plan the IP address scheme in advance. Make sure you know which addresses will be for the servers, gateways, switches etc. Don't forget printers and anything else with an IP address. (See below for some hints on an IP address scheme)
2. Change the IP address settings (DNS, Gateway etc) on each device and then shut down (power off) immediately. (see below for why we do this at this point)
3. If you are using managed switches, reset them to clear out their IP cache.
4. Bring up the gateways/routers.
5. Bring up the main DC only. Once it has settled and logged in - flush out the DNS, WINS etc so that they don't contain old data. Be careful if it is an Active Directory domain that you only delete or change entries for computers. There are a lot of other settings that you shouldn't touch.
6. Bring up the other domain controllers. Check that they can ping each other.
7. Adjust DHCP for the new scheme. Start to bring up the other core servers - again checking for connectivity.
8. Finally bring up the clients.

The key thing is to ensure that the DNS settings are correct - so change them just before the machines go down. Windows will populate the DNS with new addresses on its own.

Things to watch for...

• Printer Ports that point to a static IP address. This might be a good time to switch to a DNS based approach for print servers. We use <printername>.printers.domain.com so we can move the printers around.
• SNMP that has been locked to IP address access
• Firewall/Routers locked to IP address access.
• Any other services that you are using that are IP address only (non SNMP monitoring for example).
• Exchange relay servers using a static IP.

Questions

Q: Why change the settings and then power down. Why not bring the machines up and then change the settings?
A: Two main reasons. The first is that on some devices a reboot is required after changing some core settings. This method gets round that problem.
The second is speed. If a Windows machine cannot connect to a domain controller immediately it will start broadcasting to try and find one. You have to wait for that time out before you can log in to the machine. You will then need to reboot the machine so that it attaches to the domain correctly.

Sample IP Address Scheme

Planning the IP Address scheme is vital. It allows you to keep track on what IP address should be attached to what device. Using a separate document can also mean that multiple people can work on the IP address change and refer to the same document.

Choosing the IP Address Range

For most small networks you have a choice of either 192.168.x.x or 10.x.x.x (where x is a number). Most people will use 192.168.x.x ranges.
However because these are commonly used, if any remote access is going to take place, avoid using 192.168.0.x, 192.168.1.x and 192.168.2.x.
A good number to use would be one of the double digits, 11, 22, 33, 44 etc as these are easy to remember and type in.

In the examples below we are using 192.168.22.x

Address Planning

Use Excel to create a spreadsheet with every number from 192.168.22.1 to 192.168.22.254. Have a number of columns, including DNS name, actual name and notes, so that you can complete the document with relevant information. This also allows you to keep track if a device has multiple IP addresses.

Address Allocation

The address allocation we usually use is based on the following. This is for a small (less than 100 nodes) network. Adjust as appropriate where you have more or less equipment.

192.168.22.1 : Primary Domain Controller - global catalog, primary DNS, DHCP etc.
192.168.22.2 : Secondary Domain controller and/or DNS server.
192.168.22.3 - 10 - additional servers as required.

192.168.22.100 backwards (99, 98 etc) : remote access dial up server.

192.168.22.101 - 192.168.22.199 : DHCP for client machines.

192.168.22.200 - 192.168.22.250 : Network attached hardware that requires static IP addresses. This can include things like printers, wireless access points etc. Usually clustered starting at either 201, 211, 221 or 231 for each type. 

192.168.22.250 - 192.168.22.253 : Network hardware - switches etc.

192.168.22.254 - Gateway - either to Internet or router to another network.