PCCSA · Question #34
Match the OSI network model Layer 3 function to the correct description.
The options field is empty ({}), so I'll explain OSI Layer 3 comprehensively so you can apply this knowledge to any matching question you encounter. --- OSI Layer 3: The Network Layer Overall Goal Layer 3's job is logical addressing and path selection - it determines how data…
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Explanation
The options field is empty ({}), so I'll explain OSI Layer 3 comprehensively so you can apply this knowledge to any matching question you encounter.
OSI Layer 3: The Network Layer
Overall Goal
Layer 3's job is logical addressing and path selection - it determines how data gets from one network to another across an internetwork. While Layer 2 moves frames between devices on the same network, Layer 3 moves packets between different networks.
Core Layer 3 Functions
| Function | Description |
|---|---|
| Logical Addressing | Assigns IP addresses (IPv4/IPv6) to identify devices across networks. Unlike MAC addresses (Layer 2), IP addresses are hierarchical and routable. |
| Routing | Determines the best path for packets to travel from source to destination across multiple networks using routing tables and protocols (OSPF, BGP, RIP). |
| Packet Forwarding | Moves packets hop-by-hop between routers toward the destination. |
| Fragmentation & Reassembly | Breaks packets into smaller pieces when they exceed a network's MTU, then reassembles them at the destination. |
| Encapsulation | Wraps data from Layer 4 into IP packets, adding source/destination IP headers. |
| Logical Network Segmentation | Divides networks into subnets using subnet masks, enabling scalable addressing. |
Why Each Function Matters
Logical Addressing - Without IP addresses, routers have no way to identify where a packet needs to go beyond the local segment. MAC addresses alone can't scale globally.
Routing - Without it, packets would have no path across different networks. Routers use routing protocols to build and maintain routing tables dynamically.
Fragmentation - If a packet is too large for a link's MTU, it must be broken up or dropped. Skipping this would cause transmission failures on links with smaller frame sizes.
Encapsulation - Layer 3 must wrap the payload with IP header information before Layer 2 can frame it. Out-of-order encapsulation breaks the entire PDU structure.
What Goes Wrong Without Layer 3
- No inter-network communication - you'd be limited to a single LAN
- No scalable addressing scheme
- Broadcasts would flood the entire internet (no subnet boundaries)
- No redundancy or path selection
Memory Tip
"Routers Route Packets" - the key device at Layer 3 is the router, and its job is moving packets (the Layer 3 PDU) using IP addresses.
A simple mnemonic for the layers: "Please Do Not Throw Sausage Pizza Away" (Physical, Data Link, Network, Transport, Session, Presentation, Application) - Network is layer 3, third from the bottom.
If you share the actual matching options, I can map each one precisely to the correct Layer 3 function.
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