Ports are doors

host:port, why 8000, what is listening. localhost as a name, not a DNS course.

You will be able to

Name the usable TCP port range 1–65535, and what ports 80, 443, and 8000 usually mean.
Read host:port in a URL and name the port when it is omitted.
List which programs are listening with ss -tlnp on Linux or WSL, or lsof on a Mac, and read the address:port column.

You have a LAN IP written down. ~/projects/first-site/index.html still sits on disk. That number finds the machine. It does not find a program.

Take the IP. After it, type a colon and 8000. You now have host:port. The host is the machine. The port is the door.

A port is a number that picks which program on that machine should answer. One address, many doors. The usual http door, the usual https door, and this file’s door can share one IP. Each one sits behind a different number. This file will use door 8000.

The numbers on the doors

Ports run from 0 to 65535. The usable range you can claim is 1 to 65535. Port 0 is reserved.

Three doors you will keep meeting:

80 is the usual door for http://.
443 is the usual door for https://.
8000 is the door this file will use.

8000 is not registered as HTTP. The registry calls it irdmi. Local servers use it by convention. People treat that convention as if it were official. It is not. The number still works. The name on the registry does not make it HTTP.

Ports below 1024 are privileged. An ordinary program cannot claim them. 80 sits below that line. So does 443. So does 22, the ssh door. 8000 does not. That is why this file will use 8000, not 80.

The browser hides two doors

A URL is the address you type in the browser. It writes the host, then a colon, then the port. The browser hides the port when it is the usual one.

Type http:// and a host with no port. The browser knocks on 80. Type https:// with no port. It knocks on 443. Any other door you must type. :8000 is mandatory.

CompareWhen the browser fills in the door

	`http://` with no port	`https://` with no port	`http://` plus `:8000`
Door the browser knocks on	`80`	`443`	`8000`
Do you type the port?	No — the browser hides it	No — the browser hides it	Yes — `:8000` is mandatory

Leave the port off and you get 80 or 443. Door 8000 you type.

Write that URL now. Start with http://. Paste the LAN IP you wrote down. Finish with :8000. You have three pieces: http://, the host, and the door. Keep that line. You will need it when the door opens. Nothing answers yet. The door is still shut. You are learning its number, not opening it.

See which doors are open

A program that is listening is waiting behind a door. List the listeners. On Linux, and in WSL, run:

ss -tlnp

Each flag has one job. -t means TCP. -l means listening. -n keeps numbers, not service names. You want to read 8000, not a name. -p names the process.

Look at the address:port column. Scan it for :8000. On a quiet machine that row is probably missing. That is expected. You have not opened the door yet. Write down whether :8000 appeared. That note is your baseline. You may see other rows. Ignore them for now. You are hunting :8000.

On a Mac. ss is a Linux tool. List TCP listeners with lsof -iTCP -sTCP:LISTEN -P -n. -P keeps port numbers. -n keeps addresses numeric.

After the next topic, ss -tlnp will grow a line like this:

LISTEN 0      5            0.0.0.0:8000      0.0.0.0:*    users:(("python3",pid=8,fd=4))

LISTEN means a program is waiting. The last field names the process: python3. Do not start that process now. Learn the column you will match.

AnatomyThe address:port column

This is the field ss prints. Click each piece.

address0.0.0.0

The address this door is written on. It is not the same writing as 127.0.0.1.

port8000

The door. This is the number you type after the colon.

Same door number on a different address is a different line. Match 0.0.0.0:8000 later, not the name localhost.

Read that column as address:port. Left of the colon is the address. Right of the colon is the door.

0.0.0.0:8000 and 127.0.0.1:8000 share a door number. They do not share an address. Do not treat those two writings as the same.

localhost is a name, not a second machine. It is written in /etc/hosts as a name for 127.0.0.1. It is not the LAN IP you wrote down. The machine reads that file before it asks the wider network. Open the file and look:

cat /etc/hosts

One machine prints this line:

127.0.0.1   localhost localhost.localdomain localhost4 localhost4.localdomain4

Yours may add or drop names. The pair that matters is 127.0.0.1 and localhost.

You can name the door. Port 8000 is the one ~/projects/first-site/index.html will use. You have listed what, if anything, is listening. Next you open that door.

Flashcards

What is a port?

A number that picks which program on a machine should answer — the door.

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Quiz

Question 1 of 3

Port 80 is not privileged. Any program may claim it, just like port 8000.

False True

Show answer

Answer: False

The opposite is true. Ports below 1024 are privileged, so an ordinary program cannot claim them. 80 sits below that line. 8000 does not, which is why this file will use 8000 rather than 80. (objective 4)

Question 2 of 3

You run ss -tlnp on a quiet machine. What are you listing, and how do you tell whether door 8000 is open?

Your answer

Show answer

Answer:

ss -tlnp lists TCP listeners and the process that owns each one. Read the address:port column. If you see :8000, that door is open. If that row is missing, nothing is listening on 8000 yet.

A full-credit answer shows: A strong answer covers listing TCP listeners with ss -tlnp (or lsof -iTCP -sTCP:LISTEN -P -n on a Mac), reading the address:port column, and treating a missing :8000 as a shut door.

The command lists listening TCP sockets and names the process. The column to read is address:port. On a quiet machine :8000 is probably absent, because you have not opened that door yet. Looking at some other column, or assuming 8000 is always there, misses the measurement. (objective 6)

Question 3 of 3

You type http:// and a host, with no port. Which door does the browser knock on?

80 443 8000 22

Show answer

Answer: 80

A bare http:// URL hides port 80, so that is the door. 443 is the hidden door for https://, not http://. 8000 is this file's door, and you must type :8000 because it is not the usual one. 22 is ssh, which a web URL does not assume. (objective 5)