Check everything in terms of DNS, make sure that there's nothing pointing back to the old IP range. DNS should flush itself at the client side fairly regularly, but also double check ipconfig /all from a couple of client machines and servers. You probably need to re-create any manually created A records in DNS, as well as clear out any old records that point to the old IP. PTR records are reverse lookup records and creating new A or CNAME records will also create the associated PTR record as well.
Make sure that there's nothing in the Connection specific DNS suffix search list other than the AD TLD. If there's anything else there, especially the ISP TLD, then you're getting DNS settings from somewhere else and it can be a bit of a bear to track down. Also make sure that DHCP is assigning everything correctly in terms of local DNS servers. If necessary, charge the customer to put a Windows DHCP server in the environment so that the router isn't trying to impose its own settings on anything.
You can also check and make sure that clients are correctly reporting to DNS by running ipconfig /registerdns on a few clients and see if the correct records appear within 15-20 minutes.
Also trying a simple nslookup and a static hostname can help you see just where systems are looking for their name registration.
Some basic sanity checks are also available here:
So you want to change your IP range? – Ace Fekay
The article is a little old, but the basics of everything are still valid, especially when considering the Global Catalgoue settings of AD and making sure they're right.
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