By default Cloudflare blocks IP addresses that are known for being sources of spam and malicious content. If any of your users is blocked incorrectly, you have the ability to add him/her to the trust list, overwriting the Cloudflare default block. In addition, if you notice spam or DDoS attacks coming from a particular IP address or range, you can block it. If an address is blocked by multiple Cloudflare users it will be blocked globally.
The Trust/Block IP List tab provides you with an interface that you can use to block or whitelist IP addresses or entire networks.
Note that if there are multiple IP's you'd like to block or allow, you can specify entire IP ranges.
For example add 192.168.0.0/16 if you want to block all requests from the 192.168.*.* network or 192.168.0.0/24 if you want to block the entire 192.168.0.* network.
If you have IP addresses added to the Trust list, you can either remove them from the trust list by pressing the Untrust button, or directly move them to the Block List by pressing Block:
On the other hand, if you have blocked any IPs or IP Ranges, you can Unblock them or directly move them to the Trust list.