A fixed address for an access list
The API only accepts what is on the list. The IP cannot change on its own.
A service that requires an authorized IP list pairs with a fixed address. You add the IP once and it stays valid for the whole period.
When the IP changes
The IP changes in two cases: on replacement, and on renewal without the option to keep the same ones. In both, update the access list the same day. Forgetting that takes the integration down with a permission error.
Have two addresses on the list, not one. When the first fails, the second takes over without waiting for a replacement.