"200 Mbps" and "unlimited" on a VPS: what the numbers mean and how to test them yourself

A plan says "200 Mbps port, unlimited traffic" and it all sounds clear. Then a one-gigabyte file takes a minute instead of forty seconds, or an email arrives about "fair use". Here is what those figures really promise, and a measurement on a live server instead of a leap of faith.

In short. A megabit is one eighth of a megabyte, so 200 Mbps is at most 25 MB/s. "Port speed" is a ceiling for your server's link; the speed you actually get depends on the other end, the distance and the load. "Unlimited" nearly always means "no meter, with conditions", and the conditions are worth reading.

Megabits and megabytes

Providers state link speed in megabits per second (Mbps), while download tools show megabytes per second (MB/s). A byte has eight bits, so you divide by 8 to get MB/s. It's easiest to do the arithmetic once and keep it in front of you:

First comes the conversion to MB/s, then how long one gigabyte takes, then how much data would pile up over 30 days if the link ran flat out around the clock. The last figure matters for the "unlimited" section below: no serious service plans for every customer to push 65 terabytes a month.

Port speed is a ceiling, not a promise

"A 200 Mbps port" describes how wide the pipe is at your server. Your download speed depends on a lot more: how fast the other side serves the file, how many hops lie between you, packet loss, distance (a latency of tens of milliseconds slows TCP down as it ramps up), and whether your server shares the link with neighbours.

Can you read the port speed from inside the server? On a virtual machine, usually not:

That is ethtool ens3 on a test VPS (your interface name may differ). A virtual network card knows nothing about the physical port, and a limit, if there is one, is normally enforced outside, on the hosting side. You can't read it from the inside, so you measure.

Measure it yourself

The simplest test is to download a big file and see how fast it came. curl can print the result itself with -w, and -o /dev/null throws the data away. We took a file of about 50 MB from an Ubuntu mirror and fetched it three times in a row:

Same file, same source, seconds apart, and the speeds were 8.4 MB/s, 61.3 MB/s and 26.6 MB/s. Multiply by 8 for megabits: roughly 67, 490 and 213 Mbps. None of those numbers is "the" right one. It shows that one run says almost nothing: the mirror is busy with other people's requests, routes change, and part of the file may already sit in its cache.

So a decent test follows a few rules:

• Run it several times and at different hours, and look at the spread, not the best figure.
• Use two or three different points, near you and far away. If everything is slow, suspect your server's link. If one point is slow, that's its problem or its route.
• Pick a file big enough: on a small one TCP never gets up to speed, and you end up measuring latency instead of the link.
• Check latency separately with ping -c 4 to the same point; to the mirror ours was 31-38 ms.
• For a clean link test between two of your own servers, iperf3 is the better tool (install it with apt, start a server on the other end). I didn't run it here.

What a real speed limit looks like

For contrast, we ran the same download with a forced cap, curl --limit-rate 3M. It behaves like a limiter on a provider's link: same file, very different result. The download took 16.1 seconds instead of roughly 1-6, and the average came out at 3,179,712 B/s, about 25 Mbps, right at the cap (3M means 3 MiB/s, or 3,145,728 B/s). One run can't show a lack of spread, but repeat it and a real limit gives itself away: the speed doesn't wander, it presses against one number. If repeated measurements show a flat shelf below the advertised figure, a limiter is somewhere in the path and it's a fair question for support.

What "unlimited traffic" means

Several different things hide behind that word, and for each plan it's worth working out which one you're looking at:

• No meter. Traffic isn't counted and you don't pay per gigabyte. This is what people usually assume.
• Fair use. No meter on paper, but the terms say that if your server keeps the link saturated, your speed may be cut or you may be asked to change plans.
• Unlimited volume, limited speed. You can move any amount of data, but the port is narrow: "unlimited" at 10 Mbps and "unlimited" at 1 Gbps are very different things.
• A volume cap with a speed drop. Past a threshold the speed falls to something small. Traffic doesn't run out, but it stops being usable.

The table above shows why "unlimited" almost always rests on the assumption that customers don't use the full width around the clock: 65 TB a month on one server is a link the provider physically can't give to everyone at once. That isn't cheating, just ordinary network economics, but it's better to know before buying than after.

What to check before you buy

• Which units the speed is written in, Mbps or MB/s (it happens, and the difference is eightfold).
• Whether the speed is guaranteed or "up to". "Up to 1 Gbps" is a ceiling, not a guarantee.
• Whether the link is dedicated or shared with other servers on the same host.
• Whether traffic is counted both ways or outgoing only, and whether incoming has its own cap.
• What happens when you exceed the terms: speed cut, a warning, a request to move plans, or suspension.
• Whether there's a trial or a refund period: the simplest check is to measure on the spot, as described above.

If you're choosing a server as a whole rather than just by network, start with how to choose a VPS. How much CPU and memory your workload needs is in CPU and RAM sizing for a VPS, and the difference between a VPS, a VDS, the cloud and a dedicated server is covered in VPS, VDS, cloud and dedicated server.

FAQ

How many MB/s is 100 Mbps?

12.5 MB/s. Divide megabits by 8. A gigabyte at that speed takes about 80 seconds, assuming the other side serves it just as fast.

Why is my download slower than the port speed?

The port is the ceiling on your side. The source server, the route, packet loss, distance and shared capacity all limit speed too. In our test the same file came down at anywhere from 8 to 61 MB/s over three runs in a row.

Is unlimited traffic really unlimited?

Usually it means "no gigabyte meter". Almost always there are fair-use conditions or a speed limit too, written into the service terms. If you plan to push large volumes constantly (video, mirrors, torrents), ask support before you buy.

How do I test server speed from the command line?

Download a large file with curl -o /dev/null -w "%{speed_download}\n" URL and multiply the result (bytes per second) by 8 to get bits. Repeat at different times. Between your own servers, use iperf3.

Summary

• A megabyte is 8 megabits: 100 Mbps is 12.5 MB/s, and 200 Mbps is 25 MB/s.
• Port speed is a ceiling, not a promise: in our test one file came down at between 67 and 490 Mbps.
• Don't trust a single run: measure several times, at different hours, against different points.
• A real limit shows as a flat shelf across repeated runs.
• "Unlimited" usually means "no meter", and the fair-use terms deserve reading before you buy.

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