Since Cloudflare acts as a reverse proxy, when it’s configured as expected, all connections to your origin webserver come from Cloudflare’s IP addresses, and that may or may not be a problem for you:
If your web application is using the originating IP of the visitor as part of its logic, it will now use a Cloudflare IP address
If you use the content of your access logs, they now contain a Cloudflare IP address as the $remote_addr
Depending on your setup, you can restore the visitor IPs in a number of ways. You can find a complete list here, or below are a few of the most popular: