Modem vs Router — What's the Difference?
A modem translates your ISP's signal into something your home network can use. A router takes that connection and shares it across all your devices. They are different boxes that do different jobs — though they're often combined into one unit.
Modem = ISP connection
Sits between your ISP's cable/phone line and your home network. Converts the signal. One device connects to it directly via Ethernet.
Router = device sharing
Connects to the modem and distributes internet access to all devices — via Ethernet ports and Wi-Fi. Handles NAT, DHCP, firewall, and Wi-Fi broadcasting.
Gateway = modem + router combined
ISPs often provide a single unit that does both. Convenient but usually lower performance than separate dedicated devices.
What a modem does
Your ISP delivers internet connectivity through a physical medium — coaxial cable (for cable internet), telephone line (for DSL or VDSL), or optical fibre. Each of these carries a signal that your home devices can't understand directly. The modem (short for modulator/demodulator) translates this signal into standard Ethernet data that other equipment can use.
Without a modem, your router has nothing to route. The modem is the entry point — it gets a public IP address from your ISP and presents a single Ethernet jack for your network equipment to connect to.
Types of modems by connection type
| Connection type | Modem type | Notes |
|---|---|---|
| Cable (DOCSIS) | DOCSIS modem | DOCSIS 3.1 required for gigabit cable plans |
| DSL / ADSL / VDSL | DSL modem | Phone line; VDSL2 supports up to ~100 Mbps |
| Fibre (FTTH/FTTB) | ONT (Optical Network Terminal) | Usually provided and owned by the ISP |
| 4G/5G home broadband | 4G/5G gateway | Usually a combined modem+router device |
What a router does
The router sits between the modem and your devices. It performs several critical functions:
- NAT (Network Address Translation) — your ISP gives you one public IP address. The router shares it across all your devices by assigning each a private IP (192.168.x.x) and tracking which device each connection belongs to
- DHCP — automatically assigns IP addresses to every device that connects
- Firewall — blocks unsolicited incoming connections from the internet
- Wi-Fi broadcasting — provides wireless access for all your devices
- QoS (Quality of Service) — on better routers, can prioritise traffic for gaming or video calls over background downloads
- DNS resolution — translates domain names (google.com) to IP addresses
Modem vs router — which causes which problems
| Symptom | Likely cause | How to confirm |
|---|---|---|
| Slow speed on all devices including wired | Modem or ISP | Connect directly to modem; call ISP |
| Slow Wi-Fi but fast wired | Router's Wi-Fi radio | Speed test wired vs wireless simultaneously |
| Speed capped well below plan on wired | Router CPU bottleneck or modem DOCSIS version | Bypass router; test direct modem connection |
| Intermittent drops for all devices | Modem or ISP line quality | Check modem event log for T3/T4 errors |
| Devices drop off Wi-Fi but internet works | Router's Wi-Fi or DHCP | Wired devices stay connected during drops? |
ISP-provided gateways — why they're often the problem
Most ISPs provide a combined modem/router (gateway) as part of the service. These devices are designed to be cheap to manufacture and easy to support across millions of customers — not to deliver the best performance. Common issues with ISP gateways:
- Underpowered CPU that adds latency under load
- Limited Wi-Fi radio quality with weak antennas
- Older Wi-Fi standard (Wi-Fi 5 or even Wi-Fi 4 on older installations)
- Inflexible firmware with limited configuration options
- No QoS, no advanced DNS options, no VLAN support
Running a speed test that shows your Ethernet result is lower than your plan speed by 20–30% is often caused by the ISP gateway's routing CPU being unable to handle the full throughput.
When to use separate modem and router
Using a separate third-party router with your ISP's modem (or putting the ISP gateway into "bridge mode" to act as a pure modem) is almost always an improvement. A dedicated home router — from Asus, Netgear, TP-Link, or a similar brand — will have a faster CPU, better Wi-Fi radios, and far more configuration options. The practical difference is most noticeable on plans of 200 Mbps and above.
Note: if your ISP provides a fibre ONT (Optical Network Terminal), you typically cannot replace it — it's the ISP's equipment and required for the physical fibre connection. You can, however, connect your own router to the ONT instead of using the ISP's gateway.