Your cable modem or coax line is the bottleneck.
On cable, a worn coax run, a corroded splitter, or a DOCSIS 3.0 modem on a DOCSIS 3.1 plan will cap your speeds well below what you're paying for. This is the most under-blamed cause of 'I pay for 600 and get 200.'
Open 192.168.100.1 in a browser. Look for downstream/upstream power and SNR. Downstream should be −7 to +7 dBmV. Upstream 35–51 dBmV. Out of range → call the ISP.
If you're renting an ISP modem, ask whether it's DOCSIS 3.1. If not, demand an upgrade (free with most providers) or buy your own.
If you've been chasing cable plant issues for months and fiber is available at your address, switching is often easier than continuing to debug.
ZIP for quick check, or full address for census-block precision.
Try any US ZIP — we query the live FCC dataset. Example: 29680, 78704, or 94110.