Many network administrators feel that ICMP is a security risk, and should therefore always be blocked at the firewall. It is true that ICMP does have some security issues associated with it, and that a lot of ICMP should be blocked. But this is no reason to block all ICMP traffic!

Can you block ICMP?

They are also regularly blocked by firewalls, which causes issues that are very hard to diagnose. ICMP is used exclusively with IPv4, and ICMPv6 only exists on IPv6. You can often get away with blocking ICMP outright and not notice anything amiss.

What happens if ICMP is blocked?

If these ICMP messages are blocked, the destination system continuously requests undelivered packets and the source system continues to resend them infinitely but to no avail, since they are too large to pass through the complete path from the source to the destination.