Just a quickie. Cisco routers can give seemingly weird (especially if you’re coming from the Windows world) traceroute outputs. This is something that I did not see documented in either the CCNA or CCNP training guides.
In Cisco routers, the codes for a traceroute command reply are:
! -- success
* -- time out
N -- network unreachable
H -- host unreachable
P -- protocol unreachable
A -- admin denied
Q -- source quench received (congestion)
? -- unknown (any other ICMP message)
For further reading refer to the IBM AIX documentation – as far as I can make out the AIX codes applies to Linux as well.