Problem scenario
You get an error about insufficient IP addresses. You have exhausted a pool of IP addresses but you do not understand why. What could be wrong when you have no spare IP address, or you get a message related to this problem when you do not think that there is a shortage?
Possible Solution
You were doing an upgrade with a "rolling" option such that there was no downtime. You may need an extra IP address (besides one per node) to allow an endpoint to become available. Just as in programming with a swap of two variables, a third, temporary variable is needed. One extra IP address may be necessary for an operation that you are performing.
There may be a file or setting (e.g., in DHCP on a router) to allocate more IP addresses. Can you decrease the lease time of DHCP to reallocate an IP address more quickly? If you cannot simply change a file or a setting, subnets may need to be reconfigured. You may need to lease a public IP address. If you were doing maintenance, some endpoints may still be using IP addresses when you do not realize it.