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How shall I check whether NIC is functiong properly?

w_tom

First, confirm the NIC's electronics can talk to your
computer via PCI bus. That is what manufacturer's diagnostics
do. Better system manufacturer's provide those diagnostics
for free. Otherwise locate the appropriate diagnostic from
NIC's web site.

That only verifies communication via PCI bus. Second,
confirm communication from NIC's electronics to network and
other NICs. Again, that diagnostic can perform test IF your
NIC is from same manufacturer as another NIC in another
networked computer.

More information: NICs have a light to report valid wire
connections. Every NIC that receives data properly reports
via indicator LED. Simple test is to connect questionable NIC
to another computer or hub. If questionable NIC's receiver is
working, then questionable NIC's LED is illuminated. If
questionable NIC's transmitter is working, then indicator LED
on other computer or hub will illuminate.

However no test is as good as a diagnostic communication
between two separate computers. Diagnostics have, for
example, identified some NICs that work until test transmits
longer packets. Diagnostic found hard problem that only
appeared to user as an intermittent failure.