2.1Network basics (IP, mask, gateway)
The essentials in 30 seconds
- The IP address identifies a machine on the network; the mask delimits the local network and the gateway is the way out to other networks.
- Private addresses (192.168.x.x, 10.x.x.x) are not routable on the Internet: NAT translates to the router's single public address.
- An address in 169.254.x.x (APIPA) signals the absence of a DHCP reply; a single affected workstation = local cause, several = central cause.
Running mini-case — Sofiane reads an ipconfig in Algiers
Sofiane, a level 1 support technician in a distribution SME in Algiers, gets a call from the shipping department: “no more Internet”. He opens a console and runs ipconfig: address 169.254.87.204, mask 255.255.0.0, empty gateway. He recognises an APIPA address, self-assigned for lack of a DHCP reply. He locates himself in the SME's network, on 192.168.1.0/24 (mask 255.255.255.0, gateway 192.168.1.1), and notes that shipping is the only workstation that fell back to 169.254 — so a local failure, not a general one. He checks the cable then forces an address renewal, and notes the reflex: diagnose from the nearest to the farthest.
Standalone case for this section: it returns in the lesson, the diagram, the script and the 48h challenge of section 2.1.
1. The IP address — the machine's identity
As soon as you go beyond the isolated workstation, everything goes through the network — and a large share of IT incidents are, at bottom, network problems. An IP address identifies a machine on the network, just as a postal address identifies a building. In IPv4, it is written as four numbers (e.g. 192.168.1.10). Without a valid address, a machine can neither send nor receive: it is “unreachable”. That is Sofiane's first reflex: look at the address before anything else.
2. The mask and the gateway — talking or going out
The subnet mask (e.g. 255.255.255.0) separates the network part from the host part: it determines which machines are “in the same local network” and can communicate directly. To reach a machine outside the local network, the data is handed to the gateway (the router or the box), the way out. A missing or wrong gateway: the machine talks to its neighbours but “has no more Internet”.
3. Private addresses, NAT and reading a failure
Private addresses (192.168.x.x, 10.x.x.x) are not routable on the Internet: the way out happens through the router's address translation (NAT), and the whole local network shares the box's single public address. Most exchanges follow the client-server model: a client asks, a server answers — which helps to locate a failure (on the client side, on the path, or on the server side). Sofiane links each anomaly to its symptom: 169.254.x.x = no DHCP address.
🔎 Concrete example — 🔎 Concrete example — Reading an ipconfig and diagnosing
A user reports “no more Internet”; Sofiane displays their configuration:
Ethernet adapter:
IPv4 Address. . . . . . . : 169.254.87.204
Subnet Mask . . . . . . . : 255.255.0.0
Default Gateway . . . . . : (empty)
Reading : 169.254.x.x + empty gateway = APIPA address (no DHCP)
Diagnosis : check the physical link, then the DHCP.Reading: the address in 169.254.x.x and the missing gateway signal that the workstation received no address from the DHCP and self-assigned an emergency address.
Lesson: a single affected workstation → local cause; several workstations → central cause (DHCP, switch, box).
Section vocabulary
- IP address
- Identifier of a machine on the network (e.g. 192.168.1.10); without it, the machine is unreachable.
Simple example: Sofiane notes 192.168.1.23 on the shipping workstation.
What it is not: a machine name or an email address, which are not IP addresses.
Reflex: locate a machine → its IP address.
- Subnet mask
- Separates the network part from the host part; defines who is “in the same network” (e.g. 255.255.255.0).
Simple example: With 192.168.1.0/24, Sofiane knows that .10 and .11 talk directly.
What it is not: a simple network port number.
Reflex: who talks directly → the mask.
- Gateway
- Exit door of the local network to reach other networks (often the router/the box).
Simple example: Sofiane pings the gateway 192.168.1.1 to validate the local network.
What it is not: the DNS server address, which translates names.
Reflex: leave the local network → the gateway.
- Private address / NAT
- Internal, non-routable addresses (192.168.x.x, 10.x.x.x); NAT translates to the box's single public address.
Simple example: The whole SME network goes out through the box's public address thanks to NAT.
What it is not: a public address assigned directly to an internal workstation.
Reflex: private inside, public outside → NAT.
- APIPA (169.254.x.x)
- Self-configuration address taken for lack of a DHCP reply: symptom of an addressing failure.
Simple example: Shipping fell back to 169.254.87.204: Sofiane suspects the DHCP.
What it is not: a normal private address received from the DHCP.
Reflex: 169.254 → no DHCP address.
In practice — In practice — Reading and interpreting a network configuration
- Display your workstation's IP configuration (ipconfig): note the address, the mask and the gateway, like Sofiane.
- Determine whether the address is private (192.168.x.x, 10.x.x.x) and consistent with the reference network.
- Spot an anomaly: 169.254.x.x, an inconsistent mask or an empty gateway.
- Link the configuration to the symptom (“no more Internet”, “no more server access”) and deduce the lead.
Step-by-step script — diagnosing a “no more Internet” like Sofiane (5 min)
1) Open a console and run ipconfig: note the address, mask and gateway. 2) Check that the address is private and consistent (network 192.168.1.0/24, gateway 192.168.1.1). 3) If the address is in 169.254.x.x and the gateway is empty: the workstation received no address from the DHCP. 4) Check the physical link (cable, switch lights), then force an address renewal. 5) Compare with a neighbouring workstation: a single workstation down → local; several → central.
Keep the sheet: sections 2.2 (DNS/DHCP) and 2.4 (ping, tracert) complete this first reading reflex.
Copy-paste template — my ipconfig reading sheet
Reference network : 192.168.1.0/24 (mask 255.255.255.0)
Gateway : 192.168.1.1
Address : 192.168.1.__ (private, consistent?)
Alert : 169.254.x.x + empty gateway = no DHCP (APIPA)
Scope : a single workstation down -> local; several -> central
Reflex : from the nearest to the farthest.An isolated workstation with no Internet
Sofiane finds a workstation on 169.254: local cause, he checks the cable then the DHCP renewal.
The whole floor down
Several workstations on 169.254: Sofiane looks at the central side — DHCP, switch or box.
Server access impossible
The address is valid and the gateway answers: Sofiane looks elsewhere than at the addressing.
The site opens by IP but not by name
The addressing is healthy: the lead switches to the DNS, covered in section 2.2.
✓ Best practices
- Diagnose from the nearest to the farthest.
- For a server or a printer, prefer a DHCP reservation over a fixed IP.
- Document the SME's addressing plan so you can find your way quickly.
✗ Common pitfalls
- Setting a fixed IP in the DHCP range → address conflict.
- Confusing “no IP” (169.254) and “no DNS”.
- Forgetting to note a fixed IP set by hand.
Key takeaways
- IP = address; mask = who talks directly; gateway = exit.
- Private addresses are not routable; NAT translates to the public address.
- Reflex: ipconfig; 169.254.x.x = no DHCP address.
- A single affected workstation → local cause; several → central cause.
48h challenge — apply it tomorrow
Like Sofiane, run ipconfig on your workstation and fill in the reading sheet: address, mask, gateway, and one sentence linking what you see to the reference network 192.168.1.0/24. If you come across an address in 169.254.x.x, explain in one sentence why it betrays the absence of a DHCP reply.
- I can explain the role of the IP address, the mask and the gateway.
- I can read an ipconfig and spot an APIPA address (169.254).
- I link an addressing anomaly to the failure it explains.
Frequently asked questions
What does an address in 169.254 mean?
A self-configuration address (APIPA): the workstation did not receive it from a DHCP. Check the physical link then the DHCP — a single affected workstation points to a local cause, several to a central cause.
Fixed IP or DHCP for servers and printers?
DHCP by default for workstations; a stable address via a DHCP reservation for servers and printers — as stable as a fixed IP, but managed centrally.
An IP address in 169.254.x.x with an empty gateway above all indicates…