Every networking device is defined by which layer it can read. That single fact tells you what it can and cannot do. A device that only reads layer 2 has no idea what an IP address is, so it can never route between networks. A device that reads layer 3 can. Work out the layer, and the capability follows.
| Device | Layer | What it does | Why it matters |
|---|---|---|---|
| Hub | 1 | Repeats every incoming bit to every other port | Obsolete — everyone shares bandwidth and collisions occur |
| Switch | 2 | Learns which MAC address is on which port and forwards only there | Each port gets full bandwidth and traffic stays private |
| Router | 3 | Forwards between different networks using IP | The only device that can move traffic off your LAN |
| Access point | 1–2 | Bridges wireless clients onto the wired LAN | Adds Wi-Fi to an existing wired network |
| Firewall | 3–7 | Permits or blocks traffic against rules | The boundary control between trusted and untrusted networks |
| Modem | 1 | Converts between digital data and the carrier's signal | How the LAN reaches the ISP |
Terminating a cable
An RJ-45 plug takes eight conductors in a fixed order. Two standards exist, T568A and T568B, and they differ only in swapping the orange and green pairs. Either works — what matters is being consistent. Both ends the same gives a straight-through cable, used for connecting unlike devices (computer to switch). One end A and the other B gives a crossover, historically needed for connecting like devices (computer to computer). Auto-MDIX on modern equipment detects and corrects the difference automatically, so crossover cables are now rarely needed, but you are still expected to know the wiring.
- Strip about 25 mm of the outer jacket without nicking the insulation on the conductors inside.
- Untwist each pair only as far as necessary — no more than about 13 mm, or interference rejection suffers.
- Arrange the eight conductors flat in the chosen standard's order, with pin 1 on the left when the clip faces away from you.
- Trim them square so all eight ends are exactly level.
- Slide them fully into the plug until each conductor is visible at the very tip, with the jacket inside the strain relief.
- Crimp firmly, then test with a cable tester — testing is the step that catches the fault before it is buried in a wall.
Wireless standards and security
| Standard | Marketing name | Band | Realistic throughput |
|---|---|---|---|
| 802.11n | Wi-Fi 4 | 2.4 / 5 GHz | Up to ~150 Mb/s |
| 802.11ac | Wi-Fi 5 | 5 GHz | 400–900 Mb/s |
| 802.11ax | Wi-Fi 6 / 6E | 2.4 / 5 / 6 GHz | Higher, and far better with many clients |
- WPA3 — current standard, strongest
- WPA2 with AES — still acceptable
- A long passphrase, not a short complex one
- WEP — broken; crackable in minutes
- WPA with TKIP — superseded and weak
- An open network with no encryption
- Hidden SSID or MAC filtering as your only defence — both are trivially bypassed
Troubleshooting from the bottom up
The most efficient way to diagnose a network fault is to work up the OSI layers, because a failure at a lower layer makes everything above it fail too. Checking DNS first when the cable is unplugged wastes a lot of time.
| Command | What it tells you | Layer |
|---|---|---|
| ipconfig /all (Windows), ip addr (Linux) | Your address, mask, gateway and DNS servers | 3 |
| ping 127.0.0.1 | Your own TCP/IP stack is working | 3 |
| ping <default gateway> | You can reach your router — local network is fine | 3 |
| ping 8.8.8.8 | You can reach the internet by IP | 3 |
| nslookup example.com | DNS resolution works | 7 |
| tracert / traceroute | Where along the path the failure occurs | 3 |
ping 8.8.8.8 succeeds but ping google.com fails, networking is completely fine and the problem is DNS. That one comparison separates a name-resolution problem from a connectivity problem in five seconds.