ARP     Address Resolution Protocol
Internet uses logical addresses, known as the Internet Protocol (IP) address.  IP addresses are four bytes long.  It is customary to write the IP addresses in decimal, each bye separated by a period (such as  At the hardware level, messages must be directed to the hardware address of the node.  Ethernet uses a six-byte hardware address, also known as the IEEE address or the ether address.  Although messages are sent to logical IP addresses, ultimately, the message must be directed to the correct hardware address.  ARP specifies a set of network messages that allow a node to investigate and discover the hardware address of another node.  More specifically, a message is broadcast to all nodes on the local area network (LAN), with the IP address.  The node that uses this IP address responds by telling its hardware address.  A node must respond to ARP messages to be visible to other nodes.  

ICMP   Internet Control Message Protocol
ICMP is a set of messages used in managing the Internet and its nodes.  Particularly, two messages “echo request” and “echo reply” are used behind the scenes to measure the responsiveness of a node.  First. an echo request message is sent to the node.  The node responds by an echo reply message.  The requester measures the time it takes for the reply to come back.  Of course, in some cases the message never comes back, indicating that the target node is off line or unavailable.  Such echoing is called “pinging.”  Nodes must respond to such pings to take part on the network.

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