Ping

ping is the basic reachability test for an IP address or host. It sends an ICMP Echo Request (type 8 on IPv4, type 128 on IPv6) and waits for the matching Echo Reply. The tool reports round-trip time (RTT), packet loss, and the order in which replies arrive. It tells you whether a host is reachable at the IP layer and how stable that path is, but nothing about whether a specific service on that host is healthy.

Every reply carries the TTL field from the IP header, which is decremented by one at each router. The value you see is what is left when the packet arrived back, which gives a coarse hint about hop distance but is not a substitute for traceroute. Packet sizes can be raised to test path MTU; if a probe larger than the path MTU has the Don't Fragment bit set, intermediate routers must reply with ICMP Fragmentation Needed, otherwise the probe is silently dropped.

IPv6 uses ICMPv6 (ping6 or ping -6 depending on platform). ICMPv6 is mandatory for IPv6 to function — Neighbor Discovery, Path MTU Discovery and address resolution all rely on it — so blocking it wholesale breaks more than just ping.

A failing ping does not prove the host is down. Many networks rate-limit or drop ICMP at the edge for policy reasons, while TCP services on the same host continue to answer normally. Conversely, a successful ping only confirms that the IP stack is reachable; it says nothing about whether HTTP, DNS or any other application is responding. Combine it with a port- or service-level check before drawing conclusions.