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 188.8.131.52).
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.
Internet Control Message Protocol
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| Ping Utility
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.