Layers 1 to 3: Physical, Data Link, Network
Unit 2: The OSI Model. Lesson 4 of 49 in NetPlus Academy.
The OSI model is a seven-layer description of how data moves from an application on one host to an application on another. The exam uses it as a common language: a "Layer 2 problem" is a switching or MAC address problem, a "Layer 3 device" is a router. Learn the lower three layers first because most hands-on troubleshooting lives there.
Layer 1, Physical
Bits on a wire, fiber, or radio. Cables, connectors, hubs, repeaters, transceivers, voltage levels, and radio frequencies. The unit of data is the bit. Problems here: bad cable, wrong connector, no link light, interference.
Layer 2, Data Link
Moves frames between devices on the same network segment using MAC addresses. Switches and network cards live here, as do Ethernet and Wi-Fi (802.11) framing. It has two sublayers: MAC (addressing and media access, such as CSMA/CD) and LLC (identifies which upper protocol is inside). Error detection with the frame check sequence happens here. Problems: MAC table issues, VLAN mismatches, spanning tree loops, duplex mismatches.
Layer 3, Network
Moves packets between different networks using logical addresses, meaning IP addresses. Routers and Layer 3 switches operate here. IP, ICMP (used by ping), and routing protocols are Layer 3. Problems: wrong default gateway, wrong subnet mask, missing route.
Layers 1 to 3: Physical, Data Link, Network, explained slowly
Why the OSI model exists
Seven layers sounds abstract, but the point is simple: it gives everyone the same vocabulary. When a colleague says "it is a Layer 2 problem," they mean switching and MAC addresses. "Layer 3 device" means a router. The exam uses that vocabulary constantly, so learn to translate.
An analogy: mailing a letter
- The letter is your data.
- The apartment number on the envelope says which person in the building gets it. That is a port number (Layer 4, next lesson).
- The street address says which building. That is the IP address (Layer 3). It never changes on the trip.
- The truck that carries it from one post office to the next is different on every leg of the journey. That is the frame and its MAC addresses (Layer 2). They change at every hop.
- The road the truck drives on is Layer 1.
Layer 1, Physical: bits on a wire
Cables, connectors, fiber, radio, voltage levels, hubs, and repeaters. The unit of data is a bit. Nothing here understands addresses. A hub is a Layer 1 device because it just repeats electrical signals out every port. Symptoms of a Layer 1 problem: no link light, a cable that is too long, interference, the wrong connector.
Layer 2, Data Link: frames and MAC addresses
A frame is the package that travels on one local segment. It is addressed with MAC addresses, the 48-bit hardware addresses burned into every network card. Switches live here: they read the destination MAC and send the frame out the right port. Ethernet and Wi-Fi are Layer 2 technologies.
Two sub-parts you may be asked about: the MAC sublayer handles addressing and who may talk when (that is where CSMA/CD lived), and the LLC sublayer labels which upper protocol is inside so the receiver knows what to hand it to. The frame ends with a frame check sequence that lets the receiver detect corruption.
Symptoms of Layer 2 problems: MAC table trouble, VLAN mismatches, spanning tree loops, duplex mismatches.
Layer 3, Network: packets and IP addresses
A packet carries data between different networks. It is addressed with IP addresses, which are logical: assigned by configuration, not burned in. Routers live here. They read the destination IP and choose the next hop. IP, ICMP (what ping uses), and routing protocols are Layer 3.
Symptoms: wrong default gateway, wrong subnet mask, a missing route.
The part that trips people up: what changes on the trip
[PC A] ---- [Router 1] ---- [Router 2] ---- [Server B] Hop 1 frame: src MAC = A, dst MAC = R1 | packet: src IP A, dst IP B Hop 2 frame: src MAC = R1, dst MAC = R2 | packet: src IP A, dst IP B (unchanged) Hop 3 frame: src MAC = R2, dst MAC = B | packet: src IP A, dst IP B (unchanged)
Every router throws away the old frame and builds a new one with new MAC addresses. The packet inside, with its IP addresses, rides through untouched (unless NAT is involved, which is a later lesson). So: MAC addresses are local and temporary, IP addresses are end to end.
Keyword translation for questions
If the question says... think layer hub, repeater, cable, connector, 1 bits, signal, link light switch, frame, MAC, VLAN, FCS, 2 hardware address, duplex router, packet, IP, ICMP, ping, 3 default gateway, subnet, routing, logical address, TTL
What to memorize
- 1 bits and cables (hub). 2 frames and MACs (switch). 3 packets and IPs (router).
- MAC changes every hop. IP stays the same.