ping sends ICMP Echo Requests and reports observed replies. It tests one control-message path to an address; it does not prove that TCP, UDP, DNS, authentication, or an application works.
Troubleshooting · Lesson 2
ping
Learn how to run bounded ping tests and interpret replies, loss, RTT, TTL, and limitations.
Running a Bounded Test
Send three IPv4 requests with a two-second per-packet timeout on common iputils implementations:
$ ping -4 -c 3 -W 2 example.com
Use -6 to select IPv6. Record the resolved address because a hostname can return several addresses and repeated runs can choose differently.
What does -c 3 request?
Sequence and Loss
icmp_seq identifies requests within the run. Missing replies contribute to observed loss, while out-of-order replies can reflect varying delay. Small samples are noisy; compare multiple bounded intervals and the application's own error rate.
Loss can occur in either direction, and ICMP rate limiting can make ping loss differ from application loss.
What can a missing icmp_seq reply indicate?
Round-Trip Time
The time field is round-trip time in milliseconds from sending the request to receiving its reply. It combines outbound delay, remote processing, and return delay. It cannot reveal one-way latency without synchronized endpoint measurements.
What does a reported time=23.7 ms measure?
TTL or Hop Limit
The displayed IPv4 TTL or IPv6 Hop Limit is the value remaining in the received reply. Without knowing the sender's initial value and return route, subtracting it does not yield an exact hop count. A change can reflect a different responder, initial value, or return path.
What is the TTL printed on an IPv4 Echo Reply?
Testing the Right Layer
If ping succeeds but a service fails, test the actual port, TLS, protocol, and request. If ping fails, inspect name resolution, ip route get, neighbor state, firewall policy, and captures before declaring the host down.
What does a successful ping fail to prove?
Lesson complete
You finished ping
You can now use ping as a bounded ICMP measurement with explicit limits.
Select the address family and record the resolved address.
Bound count and wait time for repeatable tests.
Interpret loss without assuming its direction or cause.
Treat RTT as two-way and TTL as a remaining value.
Test the real application separately.
Keep your learning progress
Create a free account to save this lesson and continue learning on any device.
Create a free account