My server is being flooded

Hello, my server is being heavily flooded by some IPs from ThePlanet, from EV1 and from other sources which I can't recognize.

Everytime it happen, I add the IP to the drop list of iptables so it solve the problem, but this is not the proper way I think. How to prevent it???

I don't know much so please let me know any details to post here.

I attached a screenshot of netstat command.

 

 

 

 

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