ICMP
ICMP is the Internet Control Message Protocol. This particular packet is best known for the ping packet. But this is just one of many functions ICMP serves on the Internet. ICMP doesn't use ports. Instead it talks to the Ethernet card at a low level. But ICMP does have types to identify itself.
ICMP type 0 is a common echo-request packet. You can use this to test network connectivity between two points. Its complement is the type 8 echo-reply. Many folks, for supposed security reasons, want to block ICMP. Although blocking type 0 is fine if you're paranoid, the fact is that unless you're talking about a flood ping, and at that from a system on an Internet backbone, it probably won't affect your bandwidth adversely at all.
However, ...
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