Ping explained: latency, packet loss and ICMP
Ping checks whether a destination responds to network probes and measures the time until each answer returns. It is a useful first diagnostic step, but it does not measure download bandwidth and cannot by itself establish that an application or website is healthy.
What the result measures
Most ping programs send ICMP Echo Request messages and wait for Echo Reply messages. The reported round-trip time, or RTT, includes the journey to the target and back. A reading of 20 ms describes one response taking about 20 milliseconds; it is not a transfer speed in megabits per second.
Results commonly include sent and received counts, minimum and maximum RTT, and an average. Repeat the test to see whether latency is stable. A small number of samples can hide intermittent trouble, while a long test can be affected by changing traffic and network conditions.
Why a host may not answer
A server or firewall may block ICMP, limit replies or give diagnostic traffic a low priority. No ping response therefore does not prove that HTTPS is unavailable. Conversely, a prompt reply does not prove that DNS, TCP connections, login or the web application itself works.
Some mobile and Wi-Fi links show variable RTT because radio scheduling, interference and power-saving modes add delay. Packet loss during a test deserves investigation, but first compare with another destination and the local router to avoid blaming a remote service for a local fault.
A useful checking order
Test the local gateway, then a stable external destination, then the affected host. Compare an IP address with a domain name to separate basic connectivity from DNS resolution. Test the actual service with a browser or an appropriate client. Traceroute can show where diagnostic responses stop, subject to its own limits.
When reporting a problem, include the time, network, destination and number of probes rather than a single impressive value. Avoid flooding other systems with frequent tests. If only one site fails while other destinations work, investigate the site and route separately from your overall Internet connection.
Jitter means variation between successive delays. It matters particularly for calls and games, where irregular response times can be disruptive even when average latency looks acceptable.