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If the server has been exposed for a long time, the attackers would have your IP and would give you a little equal. To avoid exposing the IP, make sure you do not have any domain/subdomain that points directly to the server. Also if you have a configured mail server you probably have SPF headers with the IP or host so they will not be filtered by the anti-spam filters. You would have to use a separate mail server.
On the other hand, once configured CloudFlare, I recommend that you do DROP to all traffic that does not come from your IP's. That is, an exclusive whitelist to these:
I'd change the IP if you can. Additionally, maybe find a host that offers DDoS protection. As @Magnite said, if they have your server's IP CloudFlare won't do much good. It's always good to have an external firewall and DDoS protection in addition.
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