The goal is to reorganize your IP address space into a more useful configuration by taking advantage of dynamic assignment of your IP addresses. 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.

WARNING! You should be thoroughly familliar with how DHCP and IP addressing works before proceeding with this set of instructions!

    1. Send a broadcast e-mail to your users announcing the network changes. Request that users shut down their workstations at quitting time on Friday.
    2. Set your DHCP server scope's lease expire time to 48 hours
    3. Identify duplicate MAC addresses
    4. Identify all devices with static (manually configured) IP address configurations
    5. Create DHCP reservations for the devices with static IP addresses.
    6. Reconfigure devices manually assigned static IP addresses to use DHCP
    7. Retire systems not capable of using DHCP if possible.
    8. Devices that should NOT be in DHCP and which should be changed manually include:
      1. The router/default gateway
      2. Network Switches with static management IP addresses
      3. Critical Servers with static IP addresses including (but not limited to):
        1. Windows Domain Controllers
        2. File Servers
        3. DNS Servers
        4. NIS/NIS+ Login Servers
        5. Kerberos and PKI Server
  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 the previously mentionied 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:
      • Network Address (the starting address)
      • Broadcast Address (ending address)
      • Default gateway address
      • DNS Servers (if you use this option, you really should)
      • VERIFY that the mask generated is correct
        NOTE: If you get the network address, mask and gateway addresses screwed up, nobody will be able to talk to anything that isn't directly on the LAN. You will not be able to access the Internet.
    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


  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 and 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.
    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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