Ping Check

A ping command can be used to check whether a computer responds to IP requests. A special ICMP packet of the type "Echo Request" is used for this purpose, to which an IP device responds with an "Echo Reply" packet. This echo functionality is colloquially called "Ping".

In addition to checking whether a host is reachable, a ping check also checks the response time in milliseconds, also known as "round trip delay". This is the time in milliseconds that elapses from the sending of a ping request to the arrival of a ping response, i.e. twice time needed to travel from source to destination plus the local processing time at the destination site.

ICMP pings are an important tool for diagnosing network problems, so most servers also respond to ICMP echo requests. If a web server is down due to overload, or perhaps even crashed, the host will often still respond to ping requests.

Conversely, if a host does not respond to ICMP ping, this does not necessarily mean that there is not a working server behind that IP address. Some server administrators deliberately block IP packets from countries with a high proportion of malicious traffic, such as China, Russia or North Korea. Or it's a Windows host where echo replies are blocked by default until a network administrator corrects this nonsense, because, from an IT security perspective, "security by obscurity" is usually not a good idea.

Ping functionality is defined for IPv4 as well as IPv6, and this IP Checker supports both protocols. The response time shown here is the round trip delay measured from a large Datacenter located in Germany.

"Echo Reply", by the way, is only one of many ICMP types, there are many more packet types that serve diagnostic or even essential transmission functions, such as IP fragmentation indication and avoidance.

References

ICMP - Internet Control Message Protocol (Wikipedia)