In an office environment with a damaged Ethernet cable that previously supported gigabit speeds but degraded to only 100 Mb capacity, the PC’s network adapter may attempt to negotiate a 1000 Mb connection and fail, preventing network access entirely.
Solution
Manually configure the network adapter to operate at 100 Mb speed:
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sudo ethtool -s eth0 speed 100 duplex full
Substitute your own interface name for eth0 as needed.
Note: This configuration is temporary and will reset after a system reboot. For persistent settings, add the command to a startup script.