Before you start

This course is the last of the networking set, so it uses words from the earlier ones. Here they all are in one place.

Devices on a network (hosts) send data in small pieces called packets. Each host has an IP address like 10.1.5.9: 32 bits, written as four numbers from 0 to 255. A router joins networks and passes each packet one hop closer to where it's going. A LAN is the local network in one home or office.

An address has a network part and a host part. A prefix like 10.1.0.0/16 names a whole network: the /16 says the first 16 bits are the network part, so every address starting 10.1. is in it. A bigger number after the slash means a smaller, more specific network: 10.1.5.0/24 is only the addresses starting 10.1.5.. Your computer sends anything for another network to its default gateway, the router on its own LAN.

A port is a number that picks which program on a host gets the data: 443 for HTTPS websites, 22 for SSH. TCP is the protocol (set of rules) for reliable connections; UDP sends quick single messages. On a LAN, devices are found by their MAC address, a hardware address on each network card; ARP is how a device asks "who has this IP address?" to learn its MAC address. DHCP is how a new device is handed its IP address and gateway when it joins.

Encryption scrambles data with a key so only the right ends can read it. HTTPS is web traffic encrypted with TLS (Transport Layer Security), and SSH is an encrypted command line on another computer. Both check who the other end is: a website proves its name with a certificate, a digital ID signed by a company browsers trust.

Two security habits come up in every lesson. Two-factor sign-in asks for something besides your password, like a code from your phone. A passkey replaces the password with a key stored on your device, which only works on the real site, so it can't be phished (tricked out of you by a fake site).