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The TCP/IP and OSI Networking Models

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The TCP/IP and OSI Networking Models

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Cisco CCNA Study Guide

By Cbrzana

Table Of Contents

The TCP/IP and OSI Networking Models

TCP/IP Layer Example Protocols
Application HTTP, POP3, FTP, SMTP
Transport TCP,UDP
Internetwork IP
Network Interface Ethernet, Frame Relay


The Application Layer: Provide services to the application software running on a computer. Defines the services the application needs to run, not the application itself. An interface between software and the network itself.

The Transport Layer: Consists of 2 main protocol options -TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). Reliable or unreliable transport of data across a network. TCP uses acknowledgements, error recovery, etc.

The Internetwork Layer: IP defines the addresses so each host computer can have different IP address, defines the process of routing (the path to take to get data from point A to point user posted image.

The Network Interface Layer: Defines how to connect the host computer to the network. Includes cabling, connectors, voltage levels, and protocols used to deliver data (Ethernet, Token Ring, etc).

Data Encapsulation

The process of putting headers and trailers around data on each individual layer.

Data Application Layer
[TCP]'''[Data] Transport
[IP]'''[TCP][Data] Internetwork
[Link Header] [IP][TCP][Data] [Link Trailer] Network Interface
Transmit Bits

Protocol Data Units (PDU)

Frame = All encapsulated data

Packet = IP header, no data link header

Segment = TCP or UDP header, but not the IP header


OSI Layers & Thier Purpose


Layer 7 (Application): Defines interface between communications software and any applications that need to communicate on a network.

Layer 6 (Presentation): Define data formats, encryption, and compression.

Layer 5 (Session): Defines how to start, control, and end conversations (sessions).

Layer 4 (Transport): Error recovery, segmentation of large data blocks into smaller ones, reassembly of small data blocks to large ones.

Layer 3 (Network): End-to-end delivery of packets, logical addressing, defines how routing works.

Layer 2 (Data link): Deliver data across one particular link/medium.

Layer 1 (Physical): Deal with physical characteristics of the transmission medium.


Benefits of OSI Layering

Easier to learn

Easier to develop

Multivendor interoperability (same networking standards)

Modular engineering (one vendor works on one layer, another works on another layer)


Cisco’s Three-Layer Hierarchical Model

Access Layer: Uses switches, point where users are connected..

Distribution Layer: Routes traffic, segments the network, translates between media types.

Core Layer: Switches traffic as fast as possible to the appropriate service


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