Networking, from the first packet.

01 / The map

The four jobs in a small network

Your laptop is an end device. A cable or Wi-Fi radio connects it to the local network. A switch connects wired devices within that network; a wireless access point connects wireless devices. A router connects this network to other networks. A home “Wi-Fi router” often combines several of these jobs in one box.

02 / Addressing

Two addresses, two different jobs

A network interface has a MAC address used to deliver a frame on a local Ethernet link. It also uses an IP address for communication across IP networks. A router removes the incoming link-layer frame and builds a new one for the next link, while forwarding the IP packet toward its destination.

A laptop at 192.168.10.25/24 can reach 192.168.10.40/24 locally. For a public website, it sends traffic to its gateway. The mask makes that local-or-remote decision possible. See the subnet boundary move →

03 / Encapsulation

The message travels inside layers

Think of an HTTP request as a message packed for travel. Transport information identifies the conversation, IP identifies the endpoints, and a local frame carries the packet across the next link. The receiver unwraps those layers to deliver the data to the browser or server.

1. 01ApplicationHTTP describes the request and response; DNS helps find the server.
2. 02TransportTCP or QUIC manages an application conversation. HTTPS commonly uses port 443.
3. 03InternetIP addresses identify source and destination across networks.
4. 04LinkEthernet or Wi-Fi carries a frame across the current local link.

These are practical TCP/IP layers. The seven-layer OSI model is another way to discuss the same journey; real protocols do not always fit one neat box.

04 / First checks

When “the internet is down”

Work from what you can observe. A successful test at one step narrows the fault; a failure tells you where to look next.

1. Check the link

   Is Wi-Fi connected or is the Ethernet link active?

2. Check IP settings

   Does the device have an expected address, mask, gateway and DNS server?

3. Test the gateway

   Can you reach the local gateway? If not, inspect the local link and IP settings.

4. Test name resolution

   If an IP destination works but a name does not, inspect DNS. A failed ping alone does not prove the destination is down.

5. Test the service

   If routing and DNS work, check the website or application and its port.

05 / Try it

Find these settings on your own device

Open a terminal while connected to a network. Find your IPv4 address, subnet mask or prefix, default gateway and DNS server. Then resolve a familiar website name. The exact values will differ from the example above.

On macOS, Network settings shows the DNS resolver; on Linux, check the network settings app or resolvectl status when systemd-resolved is installed. A result from example.com shows name resolution, not that every website will load.

06 / Keep handy

Six words you will hear every day

Link

The cable or radio connection that carries signals between nearby devices.

Frame

A unit sent over a local link. Ethernet frames include source and destination MAC addresses.

Packet

A unit routed by IP. Its source and destination IP addresses identify the communicating endpoints.

Subnet

A group of IP addresses treated as local according to a prefix or subnet mask.

Default gateway

The router a device uses when the destination is outside its local subnet.

Port

A transport-layer number that helps a host deliver traffic to the right application, such as HTTPS on 443.