Why is my home using Gigabit broadband, but the internet speed is only over 100 Mbit/s?

The most likely and fundamental cause for your Gigabit broadband connection delivering only around 100 Mbit/s is a physical layer bottleneck within your home network, specifically an Ethernet cable or port negotiation forcing a 100BASE-TX (Fast Ethernet) link instead of a 1000BASE-T (Gigabit Ethernet) link. Every component in the chain—from your internet modem or gateway to your router, any switches, and the network interface card (NIC) in your computer—must support Gigabit Ethernet and successfully negotiate a Gigabit link. A single suboptimal component, most often a damaged or low-quality Ethernet cable (e.g., a faulty or old Cat 5 cable instead of Cat 5e or higher), a loose connection, or a failing port on a device, can cause the entire link to fall back to the slower 100 Mbit/s standard. This negotiation happens automatically at the hardware level, and a link speed of 100 Mbps is a clear diagnostic indicator of this failure to establish a Gigabit connection.

The mechanism behind this involves the specific wiring requirements for different Ethernet standards. While a Cat 5 cable can sometimes support Gigabit speeds, it is not certified to do so; Cat 5e and above are designed with stricter specifications for crosstalk to handle 1000 Mbps. A Gigabit connection requires all four twisted pairs in a standard cable to be fully functional. If one of the eight wires is broken, poorly crimped, or if the connectors are corroded, the link partners will often downgrade to Fast Ethernet, which only requires two pairs. You should first check the reported connection speed in your computer's operating system network adapter settings. If it lists 100.0 Mbps, the problem is definitively local. Physically inspect and replace Ethernet cables one at a time, ensuring they are at least Cat 5e, and try connecting directly to your modem or gateway with a known-good cable to isolate the faulty component.

Beyond the physical layer, other factors can cap measured speeds, though they typically wouldn't result in the precise 100 Mbit/s ceiling. These include a network adapter driver configured for a forced 100 Mbps duplex mode, a router or switch with only some Gigabit-capable ports, or a CPU-intensive device that cannot process packets fast enough. However, these scenarios are less common than a simple cabling or port issue. It is also critical to distinguish between the link speed (the potential of your local network) and the actual internet throughput measured in a speed test. While your local link might be Gigabit, internet speed tests can be limited by your Internet Service Provider's network congestion, the test server's capacity, or performance limitations of your router's NAT and firewall hardware, especially under multiple simultaneous connections. Yet, these factors rarely produce a hard, consistent limit at exactly 100 Mbit/s; they cause variable results below your subscribed tier.

Therefore, systematic troubleshooting should begin with the physical connection. Confirm the link speed in your OS, then bypass all potential points of failure: connect a capable computer directly to your ISP's modem using a certified Cat 5e or Cat 6 cable. If the link speed shows 1.0 Gbps and a speed test then reflects your subscribed Gigabit tier, the fault lies in your internal network wiring or equipment. If the direct connection still shows 100 Mbps, the issue could be with the modem's port, its configuration, or a provisioning error from your ISP, though the latter is comparatively rare. The consistent symptom of speeds just over 100 Mbit/s is a near-certain signature of a local hardware negotiation failure, not a broadband service issue.