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Echo Request and Echo Reply
Echo Request and Echo Reply query messages provide the ability for one system to see if another system is on the network and functional. Any Echo Request message received by a system must be responded to with an Echo Reply query message.
ICMP Message TypeThe Message Type for Echo Request is 8. The Message Type for Echo Reply is 0.
ICMP Message Codes
Neither the Echo Request nor Echo Reply query messages uses any Codes, and this field must be zero. However, some vendors (such as Cisco) use some message codes for specific services. If you see a non-zero value in this field, check the documentation for the sending system to determine what purpose it serves.
Both the Echo Request and Echo Reply query messages use three additional fields. Table 5.12 lists these fields, their size (in bytes), and their usage.