What are the ping command indicators on Cisco IOS?

Question

Accepted Answer

! = echo reply received (success); . = timed out waiting for a reply (problem on the path); U = a router returned an ICMP Type 3 "destination unreachable".
Verify Connectivity with Ping:
The ping command is the most effective way to quickly test Layer 3 connectivity between a source and destination IP address.
IOS Ping Indicators:

Indicator
Meaning

!
Exclamation mark indicates successful receipt of an echo reply. Validates Layer 3 connection between source and destination.

.
Period means that time expired waiting for an echo reply. A connectivity problem occurred somewhere along the path.

U
Uppercase U indicates a router along the path responded with an ICMP Type 3 "destination unreachable" message. Possible reasons: router doesn't know the direction to destination network, or host not found.

Note: Other possible ping replies include Q, M, ?, or &. However, the meaning of these is out of scope here.
Windows vs IOS:

Windows 10 sends 4 ICMP echo messages and expects 4 replies
IOS sends 5 ICMP echo messages and displays an indicator for each

Go deeper:

Wikipedia — ICMP — the echo (Type 8/0) and destination-unreachable (Type 3) messages behind the !, ., and U indicators.

Indicator	Meaning
!	Exclamation mark indicates successful receipt of an echo reply. Validates Layer 3 connection between source and destination.
.	Period means that time expired waiting for an echo reply. A connectivity problem occurred somewhere along the path.
U	Uppercase U indicates a router along the path responded with an ICMP Type 3 "destination unreachable" message. Possible reasons: router doesn't know the direction to destination network, or host not found.