2.1Network basics (IP, mask, gateway)
The moment you go beyond a single machine, everything runs over the network — and a huge share of IT incidents are, at heart, network problems. An IP address identifies a machine on the network, like a postal address, so data can be sent and received. In IPv4 it looks like four numbers (e.g. 192.168.1.10). The subnet mask (e.g. 255.255.255.0) decides which part of the address is the network and which is the host: it is what determines which machines are « on the same local network » and can talk to each other directly. The default gateway (usually the router) is the « exit door »: to reach a machine outside the local network — anything on the internet — the data goes through it.
Two more ideas complete the picture. Private addresses (192.168.x.x, 10.x.x.x) are used inside local networks and are not routable on the internet; the crossing to the outside happens through NAT on the router, which lets a whole local network share the box's single public address. The client-server model shapes most exchanges: a client (your browser, your PC) asks, and a server (a website, a file server) answers. For a technician these ideas are intensely practical: understanding that a machine with no valid IP — or a wrong mask or gateway — cannot communicate explains most « I have no internet » and « I can't reach the server » calls. Reading a machine's IP config and spotting an abnormal address (like a 169.254.x.x, meaning no address was received from DHCP) is a foundational diagnostic reflex.
Section vocabulary
- IP address
- The identifier of a machine on the network (e.g. 192.168.1.10) used to send and receive data.
- Subnet mask
- A value (e.g. 255.255.255.0) that separates the network part from the host part — it defines who is on the same local network.
- Default gateway
- The exit point of the local network (usually the router) used to reach machines outside it.
- Private address / NAT
- Internal addresses not routable on the internet (192.168.x.x, 10.x.x.x); NAT translates them to the public address.
- Client-server
- A model where a client requests a service and a server provides it.
What does an IP address represent?
In practice — Read a network configuration
- On a machine, display the IP configuration (ipconfig): note the IP address, mask and gateway.
- Decide whether the address is private (192.168.x.x, 10.x.x.x) and consistent with the network.
- Spot an anomaly: a 169.254.x.x address (no address received), or a wrong mask or gateway.
- Link the config to the symptom: an invalid IP or wrong gateway explains « no internet » or « no server access ».
Key takeaways
- IP = the machine's address; mask = splits network/host (who can talk directly); gateway = exit to the outside.
- Private addresses (192.168.x.x, 10.x.x.x) are not routable on the internet; NAT translates to the public address.
- Client-server: the client asks, the server answers.
- Diagnostic reflex: read ipconfig; a 169.254.x.x address means no DHCP address was received.
Frequently asked questions
What does an address starting with 169.254 mean? I see it a lot when troubleshooting.
A 169.254.x.x address is an automatic self-assigned address (APIPA): the system gives it to itself when it fails to get an address from a DHCP server. In other words, the machine tried to join the network, asked for an address, got no answer, and fell back to this « emergency » value. The immediate consequence is that it can barely communicate — no proper network, no valid gateway, so no internet and no server access — which is why this symptom accompanies a « nothing works » report. The good news is the causes are limited: check the physical link first (unplugged or faulty cable, disabled port, bad Wi-Fi), then confirm the DHCP server is up and reachable, then check the switch and the machine's network settings (it should obtain the address automatically). One machine on 169.254 usually means a local problem; several at once points to a central one (DHCP down, address pool exhausted, a failed switch).
Should I set fixed IP addresses or let DHCP do it?
Both coexist, and the choice depends on the machine's role. DHCP (automatic assignment) is the default for the vast majority of workstations: it avoids manual management, prevents conflicts, adapts to devices coming and going, and centralizes the network settings. A fixed (static) address makes sense for machines that must be reachable at a stable, predictable address — servers, network printers, and the network equipment itself. There is an elegant middle ground: a DHCP reservation, where the DHCP server always hands the same address to a given machine (identified by its MAC), giving you a stable address that is still managed centrally. Rule of thumb: DHCP for workstations, a stable address (static or reserved) for servers, printers and network gear — and always document the addressing plan.