Suppose you have an environment in which the members of your MongoDB replica set have IP addresses that do not fit in a tight CIDR range. In this case, you will need to list each IP address individually in the security.clusterIpSourceWhitelist configuration.

This also means that when you add a new node, you must first restart every other node so that the other nodes pick up the new value for security.clusterIpSourceWhitelist and allow connections from the new node. This in turn implies that adding a new node will also trigger an election. Some customers wish to minimize elections.

https://docs.mongodb.com/manual/reference/configuration-options/#security.clusterIpSourceWhitelist