A modem converts your provider signal into Ethernet for one connection. A router builds your network, hands out addresses and routes packets. One box normally does both, plus a switch and a wireless access point, which is why fault descriptions are so vague.
- The modem faces the ISP. The router faces your devices
- A modem serves one connection. A router shares it among many
- Wifi is a third function, an access point, not the router
- Bridge mode turns off the router half so you can use your own
- Two routers in series is double NAT, and it breaks inbound traffic
On this page
The modemWhat a modem actually does
Modem is short for modulator-demodulator, and the name describes the whole job. The line from your internet service provider, whether it is cable, fiber or DSL, does not carry Ethernet frames. It carries a signal shaped for that medium, and the modem converts between that signal and the Ethernet your devices speak.
Each medium carries data in its own way. Cable carries it as radio frequency signals along coaxial, DSL as tones across a telephone pair, and fiber as pulses of light. The modem modulates outbound data onto that signal and demodulates the inbound signal back into frames, which is where the name comes from.
That is the entire function of a modem, and it has three consequences worth knowing.
Modems are specific to the technology. A cable modem cannot be used on a fiber line and a DSL modem cannot be used on a cable line, because each is built for one kind of ISP signal. This is why moving house often means new hardware.
It serves one connection. A modem gives you one Ethernet handoff, enough for one computer. To connect multiple devices to the internet, something has to share that connection, and that something is a router.
It does not make decisions. A modem on its own does no addressing, no filtering and no routing, so it adds no security of any kind. It converts and passes on, which is why a modem alone gives one connected device a public address directly from the ISP.
Fiber lines complicate the vocabulary slightly. The box a fiber ISP installs is an optical network terminal, an ONT, and it does the same conversion job that cable and DSL modems do. Everyone calls it a modem anyway.
The routerWhat a router actually does
Routers join two networks and forward packets between them. In a home or small office, those two networks are the ISP network and your local area network, and joining them is what connects multiple devices to the internet at once.
It routes. Every packet leaving a device is compared to a routing table. Anything for your own network stays local, and anything bound for the internet goes to the ISP. That decision is what makes a router a router, and it is described in more detail under default gateway.
It hands out addresses. The DHCP server in the router gives every device a private address, a subnet mask, a gateway and DNS servers, which is why devices that connect to your home network reach the internet without anybody configuring them.
It translates addresses. The ISP gives you one public address and you have twenty devices. Network address translation rewrites the source address on the way out and reverses it on the way back, which is what lets all twenty devices share one internet connection.
It filters. Routers with a firewall drop unsolicited inbound traffic by default. That security is a side effect of address translation as much as a deliberate policy, and it is the reason a home network is not directly reachable from the internet.
Wifi is not on that list on purpose. Wireless is the job of an access point, and the fact that it usually lives in the same box as the router is exactly the confusion this page exists to clear up.
The difference between modem and router is a difference of function, not of hardware, and one case can hold both. Devices connect to the access point, and the access point connects them to the network that reaches the internet.
The combined boxThe combined box, and why it matters
The internet box on the wall of most homes and small offices contains four separate things, which is why the difference between modem and router is so hard to see from the outside.
| Function | What it does | What it is called on its own |
|---|---|---|
| Modem | Converts the ISP signal to Ethernet | A modem, or an ONT on fiber |
| Router | Routes, translates addresses, hands out DHCP | A router |
| Switch | The Ethernet sockets for wired devices | A switch |
| Access point | The wifi | A wireless access point |
Combining all four is sensible for a home network. It becomes a problem in three situations.
Diagnosing a fault. "The internet is down" is four possible failures inside one box. The useful question is whether the modem still has a signal from the ISP, and the box usually says so with a light before you touch anything else.
Wanting better wifi. The access point sits wherever the ISP cable enters the building, which is often a cupboard in a corner. Wifi coverage is a placement problem, and a computer far from that cupboard connects badly no matter which features you paid for.
Adding your own firewall. A business that buys a proper firewall for security now has two routers in series, both translating addresses. That is double NAT, and it breaks inbound connections, some VPNs and anything relying on port forwarding.
Bridge modeBridge mode, which is the fix for most of it
Bridge mode tells the combined box to stop being a router and behave as a modem only. It stops translating addresses, stops handing out DHCP, and passes the ISP public address straight through to whatever you connect to it.
That single setting resolves the double NAT problem, gives your own firewall the public address it expects, and puts routing and security under your control rather than the ISP's.
Two things to know before switching it on. The wifi in the ISP box usually stops working, because it depended on the router function, so you need your own access point ready before any device loses its connection.
And some ISPs hide the setting, name it something else, or require a phone call to their service desk, which is worth checking before the change is scheduled rather than during it.
The alternative, where bridge mode is not available, is to put the ISP box into a mode that forwards everything to your firewall, sometimes called DMZ host or IP passthrough. It is less clean and it usually works.
SpeedWhere the speed of a connection is actually decided
The difference between modem and router matters most when somebody is unhappy with speed, because each function caps a different part of the path and only one of them is the internet service itself.
| Link in the chain | What it limits | How you tell |
|---|---|---|
| The plan from the service provider | The ceiling for everything behind it | The provider's own service page |
| The line and its signal quality | Whether the plan's speed is reached at all | Signal levels and error counts on the modem status page |
| The modem standard | What the modem can carry regardless of plan | The model against what the provider requires |
| The router | Throughput across the internet connection | A wired speed test at the router |
| The access point and the radio | Speed for wireless devices only | A wired device compared with a wireless one |
| The switch port | Speed for one wired device | The negotiated link speed on that port |
A speed test run on a laptop over wifi measures all six at once and reports one number, which is why it settles nothing. Test from a wired device first. If the wired result matches the plan and the wireless one does not, the problem is the access point.
Upstream is the number a business should read twice. Cable plans are asymmetric, with far more capacity downstream than up, and a home streaming video never notices. An office running cloud backups, hosted phones and video calls lives on the upstream, so check that figure on the provider's plan before signing.
PitfallsWhere people go wrong
Buying a router to fix a slow internet connection. If the modem is negotiating a poor line, new routers change nothing. Check the signal levels and the ISP service status first.
Calling the whole box a modem, or a router, and then troubleshooting the wrong half. Name the function that failed. No signal from the ISP is the modem, devices with no address is the router, poor coverage upstairs is the access point.
Running two routers in series without noticing. An ISP box plus your own firewall means double NAT unless one of them is bridged. The symptom is that everything outbound reaches the internet and nothing inbound connects.
Assuming wifi is the router rather than a separate function. They are different functions that happen to share a case. Moving wifi to a properly placed access point is often the single biggest improvement available to a small office network, and it changes nothing about the internet connection itself.
Rebooting the router when the modem is the problem. Reboot the box that faces the fault. If the ISP signal light is off, restarting the wifi achieves nothing.
Keeping the ISP box as the only security in a business. It is built for a home network. A business wants a firewall with logging, VPN and policies, and the ISP box bridged in front of it.
ComparisonFour functions, and which one to blame for what
| Criterion | Modem | Router | Switch | Access point |
|---|---|---|---|---|
| Faces | The ISP | Both sides | Your devices | Your devices |
| Main job | Signal conversion | Routing between networks | Forwarding frames locally | Wireless to Ethernet |
| Hands out addresses | No | Yes, by DHCP | No | No |
| Number of connections | One | Many | Many | Many |
| Provides wifi | No | Only if combined | No | Yes |
| Failure looks like | No internet at all | Devices get no address | Some ports dead | Poor coverage |
| Can you supply your own | Sometimes | Yes | Yes | Yes |
| Adds security | None | Yes, a firewall | None | Only the wifi key |
The row about supplying your own is the practical one. In most cases the ISP owns the modem and you can replace everything else, which is why the strongest small office network is a bridged ISP box in front of equipment you chose.
FAQFrequently asked questions
What is the difference between modem and router?
The modem connects you to your internet service provider and converts the ISP signal into Ethernet, for one connection. The router builds a network on your side, gives every device an address, and decides where packets go. They face opposite directions.
Do I need both a modem and a router?
Yes, if more than one device needs to connect to the internet. A modem alone serves a single device with one public address. In most homes both functions are inside one box already.
Is my ISP box a modem or a router?
Almost always both, plus a switch and a wireless access point. That is why "the router is broken" is four different faults sharing one name.
What is bridge mode?
A setting that turns off the router function in the combined box so it acts as a modem only, passing the public address through to your own firewall or router.
What is double NAT?
Two devices translating addresses in series, which happens when the internet box and your own router both route. Outbound traffic reaches the internet and inbound connections, port forwarding and some VPNs break.
Can I use my own router with the ISP modem?
Usually yes. Put the ISP box in bridge mode or use its passthrough setting, then connect your own router or firewall to it.
Does the router or the modem provide wifi?
Neither, strictly. Wifi comes from a wireless access point, which is normally built into the same box as the router and the modem. Separating it is the usual fix for poor coverage in a home or office.
Which one do I reboot?
The one facing the fault. No internet signal at all is the modem. Devices without addresses is the router. Weak wireless upstairs is the access point, and rebooting nothing will move it.
Will a better router make my internet faster?
Not beyond the speed the ISP line delivers. Better routers improve wireless throughput, handle more devices at once, and stop being the bottleneck, but they cannot exceed what the modem receives.
What is an ONT?
An optical network terminal, which is what a fiber ISP installs. It does the same conversion job modems do, on light rather than on a copper signal.
Do businesses use the same equipment?
The functions are the same and the split is different. A business normally bridges the ISP box and puts its own firewall behind it for security and logging, with separate access points placed for coverage.
Why does the ISP box only give me one usable Ethernet port sometimes?
Because it is in bridge mode, or set to pass a single public address through, so only one device can connect. That is the modem behaving as a modem, which is the intended result.
Is a gateway the same as a router?
In this context yes. ISPs call the combined unit a residential gateway, and default gateway is a separate idea meaning the address your devices send traffic to when the destination is not on the local network.
Keep readingRelated concepts
Read next · Addressing What Is DHCP? The service in the router that gives every device an address, and the one that has failed when nothing can reach anything. Open this next9 min- Addressing · 10 min Default Gateway The address every device sends traffic to when the destination is somewhere else, which is the router doing its actual job.
- Remote access · 11 min NAT Traversal, and Why One Way Audio Is Always the Same Bug Double NAT is two of these in series, and this is what address translation does to inbound connections.
- Fundamentals · 9 min Switch vs Router, and Where a Layer 3 Switch Fits Switch vs router: which one adds ports to a network and which one reaches the internet.
- Addressing · 11 min IP Masquerading, and the One Case Where SNAT Is Better Where the second layer of translation comes from.