How ping command works

• • The Internet Ping program works much like a sonar echo-location, sending a small packet of information containing an ICMP ECHO_REQUEST to a specified computer, which then sends an ECHO_REPLY packet in return. 
  • The IP address 127.0.0.1 is set by convention to always indicate your own computer.
  • Therefore, a ping to that address will always ping yourself and the delay should be very short. This provides the most basic test of your local communications.
  
  
  The ping command is a very common method for troubleshooting the accessibility of devices. It uses a series of Internet Control Message Protocol (ICMP) Echo messages to determine:
  • Whether a remote host is active or inactive.
  • The round-trip delay in communicating with the host.
  • Packet loss.
  The ping command first sends an echo request packet to an address, then waits for a reply. The ping is successful only if:
  • the echo request gets to the destination, and
  • the destination is able to get an echo reply back to the source within a predetermined time called a timeout. The default value of this timeout is two seconds on Cisco routers.