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The OSI Model: Layers 4, 3, and 2 (Transport, Network, Data Link)

CCNA: 0 to Hero - Comprehensive Network Engineering Bootcamp

Lesson 4: The OSI Model: Layers 4, 3, and 2

We now move into the core network layers responsible for delivery.

Layer 4: Transport Layer

  • Purpose: Manages end-to-end communication between processes (applications). It segments data and provides reliability.
  • Protocols: TCP (Transmission Control Protocol - connection-oriented, reliable) and UDP (User Datagram Protocol - connectionless, faster).
  • Key Concept: Port Numbers (used to identify specific applications).
  • Data Unit: Segment (TCP) or Datagram (UDP).

Layer 3: Network Layer

  • Purpose: Provides logical addressing (IP addresses) and determines the best path for data delivery across different networks (routing).
  • Protocols: IP (IPv4 and IPv6), ICMP.
  • Key Device: Router.
  • Data Unit: Packet.

Layer 2: Data Link Layer

  • Purpose: Provides physical addressing (MAC addresses) and handles data framing, error detection, and local delivery within a single network segment.
  • Protocols: Ethernet, PPP, HDLC.
  • Sublayers: Logical Link Control (LLC) and Media Access Control (MAC).
  • Key Device: Switch.
  • Data Unit: Frame.