A two-line runbook
Start, stop, the three usual breaks, and what hosting actually sells you.
You will be able to
- Fix
Address already in useby finding the owner withssorlsofand stopping it. - Explain why
python3 -m http.server 80hitsPermission deniedwhile port 8000 works. - Work the three usual causes of "my phone still can't reach it" in order, and write a two-line runbook that starts and stops the server.
The page already loads on the phone.
http://192.168.x.x:8000 works.
Keep that.
Open a terminal and type these two commands:
cd ~/projects/first-site
python3 -m http.server 8000That is how you start. A runbook is a short list of commands you keep, so you do not have to invent them again.
When you are done, press Ctrl-C in that same terminal.
The server prints Keyboard interrupt received, exiting.
That is how you stop.
Write both lines down. That is the whole runbook.
Start from ~/projects/first-site, on port 8000.
Stop with Ctrl-C.
If start prints OSError: [Errno 98] Address already in use, a leftover server still holds the door.
Skip to the next heading. You will free it, then come back.
If it started, open the phone again at http://192.168.x.x:8000.
The page is still there.
Press Ctrl-C.
Start from the runbook again.
The page comes back.
That is what repeatable means.
When start fails
Two errors look fatal. Neither one is.
Start the server a second time, in a new terminal, while the first one is still up. Linux prints:
OSError: [Errno 98] Address already in useOn a Mac the number is 48, not 98. The wording is the same.
This is not a broken machine.
The door is already taken.
A leftover python3 from last time is the usual owner.
Find the owner. Then stop it.
ss -tlnpLook for :8000.
The process column names what holds the door.
On a Mac. There is no
ss. Uselsof -iTCP -sTCP:LISTEN -P -nand look for:8000.
If the owner is your leftover server, go to its terminal and press Ctrl-C.
Then start again from ~/projects/first-site.
The other error is a different door.
python3 -m http.server 80That prints:
PermissionError: [Errno 13] Permission deniedPorts below 1024 are privileged. Only a privileged process may claim them. 80 sits below that line. 8000 does not. That is why 8000 works and 80 does not. Stay on 8000.
On a Mac. Treat privileged ports as a Unix convention. Linux documents the line at 1024. Do not look for a Linux setting on macOS.
The phone still cannot reach it
Work the three usual causes in order. Each check kills a whole class of failure.
Put the checks in the order that eliminates a class of failure each time.
- Is the server running?
- Is it serving the right directory?
- Is the phone using the LAN IP, not localhost?
- Is a firewall blocking the door?
Is the server running? If the terminal is gone, the server is gone. Start it from the runbook.
Is it serving the right directory?
The server serves the folder you stood in when you started it.
Start from anywhere else and the phone sees that folder instead.
If you started from ~, it will not see ~/projects/first-site/index.html.
Stop with Ctrl-C.
Start again from ~/projects/first-site.
Is the phone using the LAN IP, not localhost?
localhost means the machine that is asking.
On the phone, that is the phone.
The URL is http://192.168.x.x:8000, the same one that already worked.
Then the firewall. A firewall is software that can block a door even when a program is listening. Check it. Do not assume it is off.
On a Mac. An alert may ask if you want to allow or deny the connection over the network or internet. Allow it. Open the firewall settings and look. This course does not claim a default.
If the laptop browser works and the phone still fails, the firewall is the next suspect.
On Windows / WSL
WSL 2 does not share the LAN by default. NAT is that default layout. NAT is still the default in 2026. Mirrored mode lets the LAN reach WSL. It is recommended, not default.
The documented fix is two steps.
Create %UserProfile%\.wslconfig if it is missing.
It lives in your Windows user folder, not inside Ubuntu.
Put this in it:
[wsl2]
networkingMode=mirroredClose every Ubuntu window. Wait about 8 seconds. Then open one again.
On Windows 11 22H2 and higher, with WSL 2.0.9 and higher, the Hyper-V firewall is on by default. You also need a Hyper-V firewall rule for inbound TCP. Microsoft’s documented example:
New-NetFirewallHyperVRule -Name "MyWebServer" -DisplayName "My Web Server" -Direction Inbound -VMCreatorId '{40E0AC32-46A5-438A-A0B2-2B479E8F2E90}' -Protocol TCP -LocalPorts 80The string in braces is the WSL VMCreatorId. Copy it character for character.
This combination — mirrored mode, a Hyper-V rule, and a phone on Wi-Fi — is documented. It is not verified end-to-end by this course.
If the phone still fails, do not grind. Use a second device that is not a phone. Or open the page in the Windows browser via localhost. That forwarding works even when the LAN path does not.
What a host sells
A 192.168 address is private: unique on your LAN, not on the internet.
A friend across town cannot load http://192.168.x.x:8000.
Their packets never find your network.
A host sells you a public address; your private one stops at your router.
You do not need that yet.
You have a two-line runbook.
You can start the server, stop it, and work the three usual breaks.
The same file is at its third address: http://192.168.x.x:8000.
Flashcards
Start command in the two-line runbook.
cd ~/projects/first-site, then python3 -m http.server 8000.
Stop the server.
Ctrl-C. It prints Keyboard interrupt received, exiting.
OSError: [Errno 98] Address already in use — what now?
Door 8000 is taken. Find the owner with ss -tlnp and stop it. The machine is not broken.
Same in-use error on a Mac. Which number, and which command?
Errno 48. Find the owner with lsof -iTCP -sTCP:LISTEN -P -n.
Why does python3 -m http.server 80 print PermissionError: [Errno 13] Permission denied?
Port 80 is privileged — below 1024. An ordinary program cannot claim it. Use 8000.
Quiz
Show answer
Answer: False
Port 80 fails with PermissionError: [Errno 13] Permission denied because ports below 1024 are privileged. An ordinary program cannot claim them. A second server on 8000 fails with Address already in use (Errno 98, or 48 on a Mac). Those are different doors and different errors. 8000 works because it sits above the privileged line. (objective 5)
Port 80 fails with PermissionError: [Errno 13] Permission denied because ports below 1024 are privileged. An ordinary program cannot claim them. A second server on 8000 fails with Address already in use (Errno 98, or 48 on a Mac). Those are different doors and different errors. 8000 works because it sits above the privileged line. (objective 5)
Show answer
Answer:
Start: cd ~/projects/first-site then python3 -m http.server 8000. Stop: Ctrl-C (Keyboard interrupt received, exiting.). Checks: (1) is the server running, (2) is it the right directory, (3) is the phone using the LAN IP, not localhost.
A full-credit answer shows: A strong answer covers starting from ~/projects/first-site on port 8000, stopping with Ctrl-C, and the three usual checks in that order (server running, right directory, LAN IP not localhost). Firewall after those three is welcome but not required. A start command from the wrong folder, or firewall as the first check, is not enough.
The runbook is start from ~/projects/first-site on 8000, and stop with Ctrl-C. The three usual causes are: server running, right directory, phone using the LAN IP not localhost. Firewall comes after those three. (objective 6)
Sample answer
Start: cd ~/projects/first-site then python3 -m http.server 8000. Stop: Ctrl-C (Keyboard interrupt received, exiting.). Checks: (1) is the server running, (2) is it the right directory, (3) is the phone using the LAN IP, not localhost.
A full-credit answer shows
A strong answer covers starting from ~/projects/first-site on port 8000, stopping with Ctrl-C, and the three usual checks in that order (server running, right directory, LAN IP not localhost). Firewall after those three is welcome but not required. A start command from the wrong folder, or firewall as the first check, is not enough.
The runbook is start from ~/projects/first-site on 8000, and stop with Ctrl-C. The three usual causes are: server running, right directory, phone using the LAN IP not localhost. Firewall comes after those three. (objective 6)
Show answer
Answer: Find who owns port 8000 with ss or lsof, then stop that process
Errno 98 means the door is taken, usually by a leftover server. ss -tlnp (or lsof on a Mac) names the owner; stop it, then start again. Port 80 is a different failure — Permission denied, not in-use. The wording sounds like a broken machine, but the owner is an ordinary process. The first server is still holding the door, which is why the second start failed. (objective 4)
Errno 98 means the door is taken, usually by a leftover server. ss -tlnp (or lsof on a Mac) names the owner; stop it, then start again. Port 80 is a different failure — Permission denied, not in-use. The wording sounds like a broken machine, but the owner is an ordinary process. The first server is still holding the door, which is why the second start failed. (objective 4)