The 100 Mbps result doesn't automatically mean your Ethernet cable is bad. On Windows 11, check Settings > Network & internet > Ethernet for the Link speed. On Windows 10, use ncpa.cpl via Run and view the Speed field in the adapter status.
On Mac, go to System Settings > Network > Ethernet > Details > Hardware. The negotiated rate shows the connection between your PC and router or switch, not your internet speed or the cable's maximum capability. A gigabit link (1 Gbps) can still show 100 Mbps if a port or adapter is limited.
Look for Cat5, Cat5e, or Cat6 markings on the cable; Cat6 supports 1,000 Mbps. Older cables may handle high speeds over short runs but falter over longer distances. If your PC reports 100 Mbps when expecting 1 Gbps, verify hardware on both ends — a 100 Mbps router port or USB adapter will cap the speed.
Reseat connectors and swap in a short, known-good cable; if speed jumps to 1 Gbps, the original cable or connectors are at fault. If unchanged, check port specs and adapter settings. For thorough testing, use iperf3: run iperf3 -s on one machine and iperf3 -c SERVER_IP on another, then compare results after cable swaps.
Source: Engadget · Summarized by HeadlinesBriefing