Allowlisting, eg configuring your system to allow access through a specified range of IPs, is not recommended. The Cloud Elements platform is a public cloud hosted by AWS, and shared IPs may change without notice; accordingly, the recipient would be allowlisting all traffic from Cloud Elements.
If you plan on using endpoints that need Cloud Elements IPs to be allowlisted, all IPs for the given environment need to be allowlisted. For example, if you are using both a staging and a US production environment, then all the IPs for these environments need to be allowlisted, but EU production IPs would not. This is because we cannot guarantee which IPs will actually be in use by our servers; accordingly, all IPs must be allowlisted.
If allowlisting by IP is the only option, the recipient domain will want to check for the most up-to-date IP information by downloading our ip-ranges file. This file will be updated whenever new IPs are added to our IP range. If a receiving server is allowlisting the range, it's recommended they periodically check to ensure they have the current range; the most recent version of the file was created on 10/14/2020.