ping command

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Internet Control Message Protocol (ICMP) is used to report errors and send messages about the delivery of a packet.

• messages are generated under error conditions in most types of unicast traffic
  • not for broadcast traffic
• can be used to test and troubleshoot connectivity issues on IP networks
• ping command sends a configurable number and size of ICMP request packets to a destination host
  • implemented on both Windows and Linux
  • can perform a basic connectivity test
    • not dependent on target host running any higher-level applications or services

Basic Usage

• ping IPAddress performs a basic connectivity test
  • output shows:
    • the message “Reply from IPAddress”
    • time it takes for server’s response to arrive
    • round-trip time (RTT) is used to diagnose latency problems on a link
    • TTL field shows the value of the counter when the packet arrived at its destination
  • Time to live (TTL) IP header field is reduced by one every time a packet is forwarded by a router
    • called a hop

ping Error Messaging

• two messages received if ping is unsuccessful:
  • destination host unreachable
    • no routing information
      • local computer does not know how to get to the IP address
      • could be caused by a configuration error on the local host
        • e.g., incorrect default gateway
  • no reply (request timed out)
    • host is unavailable or cannot route a reply to your computer
    • requests time out when the TTL is reduced to 0 because
      • the packet is looping
      • congestion causes delays
      • when host does not respond

ping Switches

• can use hostname or fully qualified domain name instead of IP address
• when pinging by name, use -4 or -6 to force tool to use IPv4/IPv6
• -t continues to ping host until interrupted (Ctrl+C)
• different syntax in Linux
  • executes until manually halted by default
  • -c switch sets a number of specified packets to send