Id the IP address is basically the same of any other poker player on the table, then it is assumed they are within the same building or sharing the same internet connection, which would be suspicious enough for the casino to limit the access of one of those poker players onto the table. Realistically, two people geographically separated could still communicate between them through the use of tools like Whatsapp and Discord, so the IP filter would not be completely effective. However, there is also a possibility not to allow gamblers to choose what table they would be playing on, but rather assign them a random table so they play against strangers, this way it becomes extremely unlikely any collusion happens.
Not completely effective is an understatement, this is not effective at all but thanks for clearing up what you meant. This is easily defeated by residential proxies and they are very cheap. IP filtering is useless. Table randomization is much better, even if it is not perfect.