What are ICMP and ping?

ICMP (Internet Control Message Protocol) carries control and error messages between devices, such as host unreachable. Ping is the simplest tool that uses it: it sends a short message to an address and waits for a reply, to check whether a device is reachable and how fast. Administrators use ping every day. Attackers use it too, to discover which addresses are alive before scanning them, and floods of ICMP messages can overload a link or a device. Many firewalls limit ICMP from the internet. NetSenX raises an alert on ICMP floods on every plan.

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