Your machine has an address

~/projects/first-site/index.html is already on disk.

Do not open it. Do not edit it. The page can wait.

This topic is about the machine that holds the file, not the file.

Open a terminal. Type this and press Enter:

ip -4 addr

A list of interfaces appears. An interface is a named connection the machine uses to talk on a network.

One line will look exactly like this:

inet 127.0.0.1/8 scope host lo

That line is loopback. Loopback is this computer talking to itself.

The address is 127.0.0.1. The /8 means the whole 127.0.0.0/8 block is loopback, not only .1.

scope host and lo mark it as this host only. Loopback must not appear outside a host.

So this is not the address you write down.

Scan the rest of the listing. Each inet line sits under an interface name.

lo is loopback. Skip it.

Find an inet line that is not 127.0.0.1.

The four dotted numbers after inet are this machine’s LAN address.

A LAN is a local area network: the machines that share your Wi-Fi.

The -4 keeps the list to IPv4. Write that address down. You will need it later.

On a Mac. There is no ip command. First run networksetup -listallhardwareports. Find the device name next to Wi-Fi — it is not always en0. Then run ipconfig getifaddr with that name. Empty output means the wrong interface, not a missing network.

On Windows / WSL. Run the same ip -4 addr inside your Ubuntu terminal.

Four numbers

An IPv4 address is four octets. An octet is eight bits. Four of them make 32 bits in total.

They are written with dots, like 192.168.1.42.

That shape is a documented example, not a capture from this machine. Your numbers will differ.

Every machine on the Wi-Fi has one of these. They are private addresses.

A private address is one that anyone may use on their own network. You do not register it first.

These three ranges are the RFC 1918 private blocks. RFC 1918 is just the name of that reservation.

These numbers are not unique on the internet. Another network may use the same four octets.

They only have to be unique on your LAN.

Read the first number of the address you wrote down.

If it is 10, the address sits in 10.0.0.0/8.

If the first two numbers are 192.168, it sits in 192.168.0.0/16.

The middle block is the trap. 172 is not enough.

The second number must sit between 16 and 31.

172.16.0.0 through 172.31.255.255 is private. A 172 with any other second number is not.

Try a shape. 10.0.0.5 starts with 10, so it is 10.0.0.0/8.

192.168.1.42 starts with 192.168, so it is 192.168.0.0/16.

Now classify yours. Tick one of the three rows.

Write the block next to the address. You need both later.

If the number you wrote is 127.0.0.1, you copied the loopback line. Look at the listing again.

If you see more than one other inet line, pick the interface you are actually using.

This computer, or the network

127.0.0.1 means this computer talking to itself.

Nothing on the Wi-Fi can use it. It never leaves the machine.

A 192.168 address is the opposite kind of local. Other machines on your network can send to it.

It still stops at your network. A private address is not reachable from the public internet.

When traffic goes out, the router rewrites the private address. That rewrite is NAT. We stop there.

The same wall applies to a 10. address and to a 172.16–31. address. Private means local.

Keep the number

You have not touched index.html. The file is still in ~/projects/first-site.

What changed is what you know about the machine that holds it.

You printed its LAN address. You placed that address in one of the three private blocks.

Copy the number somewhere you will not lose it.

Do not copy 127.0.0.1. If that is what you have on paper, run the command again.

Next you will look at the doors on that address. An address names the machine. Something still has to be listening.

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