Capturing IP Addresses really depends on where your site operates and the legal needs that you need to fulfil, even replacing the last Octet may not be enough for you... you will need to check with your own legal team on that.
That said, have you considered working with your developers to create logic based on the IP Address and just have them pass a simple "true" or "false" if the IP is considered internal? Rather than trying to collect the IP address and create logic on it later?
Classifications would of course allow you to make changes retroactively, but not collecting IP information into your dimensions would avoid a lot of potential legal issues....
If you really just need a way to segment your internal traffic from your external traffic (and not exclude internals outright), then maybe this would be a future-proof viable option.