IP Renumbering Plan (using DHCP)

OVERVIEW

The goal is to reorganize your IP address space into a more useful configuration by taking advantage of dynamic configuration. This option is the best one to use if the majority of your computers are on DHCP already. Renumbering always requires manually configuring a large number of machines. This process is intended to minimize the effects and the work involved in performing the migration.

1. MONDAY
   1. Send an e-mail announcing the change and requesting that users shut down their PC's at quitting time on Friday.
   2. Set DHCP lease expire time to 24 hours
   3. Identify duplicate MAC addresses
   4. Identify all devices with manual IP configurations
   5. Create DHCP reservations for the manual devices
   6. Reconfigure devices assigned IP's manually to use DHCP
   7. Retire systems not capable of using DHCP if possible.
   8. Devices that should NOT be in DHCP should be changed manually
      1. The router/default gateway
      2. Critical Servers (Domain Controller, Login Server, Kerberos server etc.)
2. WEDNESDAY: Steps 1-5 should be complete.
3. Allow your current DHCP leases to expire.
4. FRIDAY: Quitting Time
   1. Have all users shut down their computers. We used a previous expiry time change to cover the following:
      1. Users not in the office (about 2% of the users)
      2. Users too stupid to find the power button (~15-20% of the users)
      3. Users too 'busy' to read their e-mail (40% of the users)
      4. The remaining personnel will have followed instructions (40-60% of the users)
   2. Change the DHCP scope settings for:
      • Starting address
      • Ending address
      • Default gateway address
      • VERIFY that the mask generated is correct
   3. Change the IP and mask on the router interface(s)
   4. Make any wiring changes to convert subnets from routed to switched networks.
5. MONDAY (next week) – TESTING & WRAPUP

DETAILED PLAN

1. MONDAY: Set DHCP lease expire time to 24 hours. This gives up to 3 opportunities over the following weekend for leases to expire and renew with new information. You can also decrease the ARP Cache timeout in the DHCP options to 2 minutes. This will greatly increase ARP traffic, but will decrease the time required to find new MAC addresses when a bad address already exists (such as for the default gateway).
2. Identify duplicate MAC addresses. Duplicate MAC addresses will cause conflicts such as being unable to communicate with a printer, server or workstation. Checking for duplicate MAC addresses is rather easy (but not foolproof) if you have a Cisco router. Simply ping the broadcast address of the subnets that will be involved in the change, then check the ARP and MAC cache information on the router using the show arp & show mac commands. Note that Windows XP and many Unix/Linux systems do not respond to broadcast pings for security reasons, thus, it is possible to miss duplicate MAC addresses.
3. Identify all devices with manual IP configurations (printers, routers, switches, and especially servers).
4. Create DHCP reservations for the manual IP devices, INCLUDING THE ROUTER (if you have not done so already).
5. Change the manual IP devices to DHCP. The router is the default gateway. At this point in the procedure, we haven’t changed any of the IP information, so leave the router statically configured. Additional devices you may wish to reconfigure to DHCP manually on Friday include Domain Controllers, switches and other critical network devices. Make sure this is a short list.
6. WEDNESDAY: Steps 1-5 should be complete. Microsoft’s default lease expiration time is 3 days. If you have not changed the default you will need 3 days for the leases to expire, which puts you in the middle of the weekend and costs you at least 1, possibly 2 opportunities for the DHCP leases to expire and renew properly with new IP information.
7. LEASE EXPIRATION: Allow all current DHCP leases to expire. Three days is the default setting in Microsoft’s DHCP manager. If you have set your lease settings to a longer period, you will need to complete steps 1-5 earlier in the week, or do them in the previous week.
8. FRIDAY: Quitting Time –
   1. Have all users shut down their computers
   2. Change the DHCP scope settings. Once all devices are on DHCP and your users have gone home for the day, change the following DHCP scope settings:
      • Starting IP address
      • Ending IP address
      • Default gateway address
   3. Change the IP and mask on the router interface(s)
   4. Make appropriate wiring changes at this time to turn the routed subnets into a single switched domain. If you are using hubs, make certain you do not violate the broadcast segment limits inherent to your network’s physical protocol (Ethernet, Token Ring, FDDI etc.). ATCC is not merging subnets, only utilizing space set aside earlier, therefore no wiring changes should be needed under this plan.
9. NEXT MONDAY: TESTING/& WRAPUP
   Reboot devices that have not changed their configuration. If everything goes according to plan, all computers using DHCP will have at least 1 opportunity per day from Thursday forward for their lease to expire and move to the 24 hour DHCP lease time. From Friday, COB forward, they will have at least 3 opportunities for their lease to expire and pick up the new address and subnet mask information before Monday morning. You may need to clear the ARP cache on the router if it is set with very long ARP expiry times (longer than a few hours). The router should automatically update the ARP and routing tables for directly connected subnets.
10. RESTORE ORIGINAL DHCP SCOPE SETTINGS. Restore the Lease Expire time back to its original setting. If you changed the ARP timeout, restore it to its original setting.

At this point, you can Move all devices previously identified as static, back to static addresses if this makes you feel more comfortable. You will need to put in their original IP address, but use the new mask and default gateway address. With DHCP reservations set, the devices should remain at the same IP, however if the DHCP server is swamped with requests and the connection times out, servers providing critical services may not respond to the network because they will have incorrect IP information.

 

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