Networking basics: how devices find each other

Networking has a reputation for being hard, but the core ideas fit on a page. Once they click, a huge share of IT problems (“the internet is down,” “I can’t print,” “the website won’t load”) start making sense.

IP addresses: every device needs one

An IP address is a device’s address on a network, like a street address for mail. The common kind, IPv4, looks like four numbers separated by dots, such as 192.168.1.25.

Some address ranges are reserved for private networks, inside homes and offices:

• 10.x.x.x
• 172.16.x.x through 172.31.x.x
• 192.168.x.x

Those addresses work only inside your network. Out on the internet, your whole home or office usually shares one public address that your internet provider gives you.

There’s also IPv6, a newer and much longer address format (like 2001:db8::25) created because the world ran out of IPv4 addresses. Many networks now run both side by side.

The subnet and the gateway

When you ran ipconfig in step 2, you saw two more numbers next to your IP address:

• Subnet mask (often 255.255.255.0): says which addresses are on your local network. With that common mask, every address starting 192.168.1. is a local neighbor.
• Default gateway: the router’s address. Anything that isn’t local, like a website, gets sent to the gateway to forward on.

DHCP: handing out addresses automatically

Nobody types IP addresses into every phone and laptop. DHCP does it automatically. When a device joins the network, it asks “can I get an address?”, and the DHCP server (usually your router) hands it an IP address, subnet mask, gateway and DNS server.

If a Windows device ever shows an address starting 169.254, that’s a tell: it asked for an address and nobody answered. Check the cable, the Wi-Fi connection, or the DHCP server.

DNS: the internet’s phone book

Computers connect using IP addresses, but people use names. DNS translates names like example.com into IP addresses. When DNS breaks, it looks exactly like “the internet is down,” even though the connection is fine. You can test it yourself:

ping 8.8.8.8
ping example.com

If the first one works and the second one fails, the connection is fine and DNS is the problem.

Routers, switches and access points

• Switch: connects devices within one network using cables. Think of it as a power strip for network connections.
• Router: connects different networks, like your home network and the internet, and forwards traffic between them.
• Wireless access point: lets devices join the network over Wi-Fi.
• Firewall: decides which traffic is allowed in and out.

The “router” most homes have is all four in one box. Businesses usually use separate devices for each job.

NAT: many devices, one public address

Your router uses NAT (network address translation) to let every device in the house share that one public address. It keeps track of which inside device started each conversation, so replies come back to the right place. A side effect: devices on the internet can’t simply reach into your network uninvited, which is part of why home networks work as well as they do.

Ports: which door to knock on

One device can run many services at once, so connections also name a port, a numbered door for a specific service. A few you’ll see constantly:

Port	Used for
53	DNS
80	Web (HTTP, unencrypted)
443	Web (HTTPS, encrypted)
22	SSH (remote command line)
3389	Remote Desktop

Putting it together: what happens when you visit a website

1. Your computer already has an IP address from DHCP.
2. It asks DNS for the website’s IP address.
3. That address isn’t local, so it sends the request to the default gateway.
4. The router uses NAT to send it out to the internet from your public address.
5. The website answers on port 443, and the reply comes back the same way.

Try this

nslookup example.com
tracert example.com

(On macOS or Linux, use traceroute instead of tracert.) The first shows DNS translating a name into an address. The second shows every router your traffic passes through on its way there.

Key takeaways

• Every device needs an IP address, a subnet mask, a gateway and a DNS server, and DHCP hands all four out automatically.
• 169.254 addresses mean DHCP didn’t answer.
• When a ping to an address works but a ping to a name doesn’t, it’s DNS.