Networking basics

IP addresses

Every device on a network has an IP address, like a house number. Inside your home, addresses usually start with 192.168. These are private and only work on your own network.

Your router has one public address that the whole house shares on the internet. It keeps track of which inside device asked for what. This is called NAT.

Ports

If the IP is the building, a port is the apartment number. One computer can run many services, each listening on its own port: websites on 80 and 443, SSH on 22, DNS on 53.

Every open port is a door. Attackers scan for open doors, so close the ones you don't use.

DNS

DNS is the internet's phone book. It turns names like google.com into IP addresses. Tools like Pi-hole run their own DNS server to block ad and tracking domains for a whole house.

Seeing it yourself

hostname -I          # your IP address
ping -c 4 google.com # can you reach the internet?
nslookup google.com  # ask DNS for an address
sudo ss -tulpn       # which ports are open here

Try it

Run the commands above on any Linux machine, then use the subnet calculator on the home page with your own IP and /24.