How to check hotspot connection limits before attaching multiple phones to a laptop server

This is a translated article. Read the original in Korean ↗

I was building a web app called formation for dance studios. Everyone in the room follows the same choreography video on their own phone, kept in sync. A laptop acts as the server and the phones connect to its address. The question was whether we could open a hotspot and connect about 10 people to it. I asked an AI coding assistant. It was a design question, not a bug, and on paper the bandwidth looked like plenty.

Conclusion

Check two things before you use a hotspot. (1) How many devices the phone hosting the hotspot can take at the same time. Ten viewers plus the server laptop all have to join the same hotspot, so the limit has to be 11 or more before the hotspot is even a candidate. This article could not confirm that number. Check it in the manufacturer's or carrier's documentation, or with a real connection test. The "5 devices" figure in the body has no source, so don't treat it as the answer (how to check is in step 1 of the Fix section below). (2) Whether one viewer phone on the hotspot can actually open the laptop server's address. If it can't, first check that the laptop's firewall allows Node.js (or the server port) on both private and public networks . If it's allowed, you've confirmed the address is right, and it still won't open, suspect client isolation (a setting that blocks connected devices from talking to each other). Bandwidth math can pass and you can still get stuck on either of these. In that conversation, only the math was done. Neither check was run, and the hotspot was dropped in favor of the studio Wi-Fi .

Bottom line: a hotspot only becomes a candidate after it passes both checks. Until then, the router (studio Wi-Fi) is the default. Nobody has measured yet how many devices that router can handle.

Situation

In this web app, a leader phone sends the playback position and the viewer phones follow it . The server runs on Node on the laptop. It reads video segments from disk and streams them out in the response . The question came up in a conversation with an AI coding assistant . The address viewers use to reach the server changes every time the network changes, so the project later moved to opening it with a QR code .

Symptom

All anyone saw on the surface was one question: "Is it OK to connect 10 people?" The math was simple. One video is about 4.5Mbps, so 10 devices come to about 45Mbps . Once the hotspot came up, the numbers were broken down by quality level: 10 people × 480p is about 11Mbps, and × 360p is about 6Mbps . Going by the math alone, the answer stops at "lower the quality and it'll fit."

Cause

The math looks only at bandwidth. On a hotspot, though, the connection itself can fail before bandwidth ever becomes the issue.

(a) Simultaneous connection count. The answer said "iPhone Personal Hotspot has long been capped at 5 devices." It added the caveat that this varies by model and carrier, and gave no source . With 10 people, that one number would leave more than half unable to connect. The number stayed unconfirmed in that conversation too.

(b) Client isolation. Some hotspots block traffic between the devices connected to them. The internet works, but the phone next to you can't reach the laptop. In this app the phones connect directly to the laptop, so if isolation is on, not a single viewer will load .

On top of that, the claims the bandwidth judgment rested on were calculations or assumptions, not measurements. That covers the line "the only variable is WiFi," the figure "measured combined 200–400Mbps," and "CPU/memory load is close to zero" .

Fix

Check these two things before you use a hotspot.

1. Find the maximum number of devices the hotspot phone allows (it needs 11 or more, counting the server laptop). Look up that model's simultaneous connection limit in the manufacturer's or carrier's documentation, or first test whether 10 viewers and the laptop can actually connect together. If you can't confirm the limit, you don't know the number, so drop that hotspot as a candidate. Treat the "5 devices" from the session as an unsourced number .
2. Connect the laptop and one viewer phone to the same hotspot. On the viewer phone, pick the hotspot name from the Wi-Fi list and enter the password. The laptop has to be on this hotspot too, or the phone can't reach the server. For the laptop's address and port, copy the "📱 뷰어 접속" ("📱 Viewer access") URL from the server page (server.html) . If that URL looks wrong, it may be because the server automatically picks the first non-internal IPv4 address, so it can grab the wrong one when the laptop has several adapters . If the laptop is hosting the hotspot, the gateway is usually 192.168.137.1. If a phone is the hotspot, the address is in the 192.168.x.x range that phone hands out . The port used in the session was 3000 . Open the resulting http://노트북주소:포트/viewer.html (laptop address : port) in the viewer phone's browser.

• If it opens, there's no isolation. Move on to the next step.
• If it doesn't open, suspect client isolation first. Also check whether the laptop's firewall is blocking the server port.
• If you can't find a setting to turn isolation off, don't use that hotspot.

Only put the hotspot back on the table once both checks pass. In the end, the decision was a judgment call: the user dropped the hotspot and went with the studio Wi-Fi . That wasn't based on a measurement, so how many devices the studio router can handle needs to be checked the same way.

Bonus — keep calculated numbers separate from measured ones

45Mbps for 10 devices is a calculation, and the "measured 200–400Mbps" was never actually measured . If you put both kinds of number in the same sentence in a write-up or a meeting, listeners will take the calculation for a measurement.

Reporter's Notes — 45Mbps checks out, but can they even connect?

The conversation from the day a hotspot was being considered instead of the studio Wi-Fi.

Park Doeun

You know that web app where 10 people in the studio follow the same choreography video on their own phones? The laptop's the server and the phones connect to it. If we open a hotspot and connect everyone, will the bandwidth hold up?

Jung Bada

The math is simple. One video needs about 4.5 megabits per second (Mbps), so 10 devices is 45. Wi-Fi should handle that

Jung Haneul

45 is right, but what's your basis for "should handle it"? Has anyone actually played it on 10 devices at once?

Jung Bada

No real measurement, no. But the AI said a 5GHz router (the faster band) gets 200–400

Jung Haneul

Was that 200–400 measured? No, it's just a number someone wrote down. The math passes, but there's no evidence.

Park Dohyun

You can always make bandwidth work by lowering the quality. The real catch was that it's a hotspot. Talking only about bandwidth dodges the actual problem.

Park Doeun

What's so different about a hotspot? Isn't it just sharing the internet?

Jung Bada

iPhone Personal Hotspot limits how many devices can connect at once. The AI said 5 max. With 10 people, not even half get in

Jung Haneul

Where did that 5 come from? It even came with a caveat that it varies by model and carrier. If you draw a conclusion from an unconfirmed number, you're only looking at one side, same as Bada was with bandwidth.

Park Doeun

So how do you check that limit? Does it show up right in the settings screen?

Jung Bada

No idea. That's still unconfirmed. We'd have to find the per-model limit in the docs, or first see whether 10 phones and the laptop actually connect.

Park Doeun

Then what's the second roadblock? What's client isolation? They connect fine, but only the laptop won't load?

Park Dohyun

Right. Some hotspots have a setting that blocks connected devices from talking to each other. The internet works, but the phone next to you can't reach the laptop's address. Our phones connect straight to the laptop, so with that on, not a single one loads

Jung Haneul

So 45 or 11, it means nothing if they can't connect. The math being right doesn't let you say "it works."

Jung Bada

Ah, yeah. I fell for the AI saying "the only variable is WiFi" too. That was based on the math alone

Park Doeun

So what did we do this time? Drop the hotspot and call it done?

Park Dohyun

We dropped the hotspot and went with the studio Wi-Fi. But that's a judgment call too, not a measurement. Nobody's checked how many devices the router can handle either

Park Doeun

If it's a judgment call, can't we just start by opening the address on one viewer phone? There's nothing in the conversation about opening that address over the hotspot.

Jung Haneul

Right. So let's check just two things first: how many devices the hotspot phone can take, and whether one viewer phone can open the laptop's address. If either one fails, there's nothing more to consider about the hotspot.

Park Dohyun

The math can pass and you can still get blocked by connection limits. Before you attach a bunch of phones to a laptop, check the device count and isolation first.