# 2.

* Communication Entity
  * Source
  * Medium
  * Destination
* Protocols
  * message encoding
  * message formatting and encapsulation
  * message size
  * message timing
  * message delivery
* Model&#x20;
  * OSI
  * TCP/IP
    * Application
    * Transport
    * Internet
    * Network Access
* Communication Process
  * Segmentation
  * Multiplexing
  * Encapsulation
  * De-encapsulation
* Protocol data unit (PDU)
  * **Data - Application layer**
  * **Segment - Transport layer**&#x20;
  * **Packet - Internet layer**
  * **Frame - Network access layer**&#x20;
  * **Bits - Physical layer**&#x20;
* Address
  * Data Link
    * Ethernet (MAC)
  * Network
    * IP (IPv4, IPv6)
* Internetwork Operating System
  * Store in Flash Memory
  * Load into Memory
* Command-line Interface (CLI)
  * Access Method
    * Out of Band
      * Console
      * AUX
    * In Band
      * Telnet
      * SSH
  * Terminal Emulation Program
    * Putty
    * Teraterm
  * Access Mode
    * User (EXEC) mode
    * Privileged (EXEC) mode
    * Global configuration mode
    * Specific mode (interface/line configuration)

Hostnames allow devices to be identified by network administrators over a network or the Internet. Passwords are the primary defense against unauthorized access to network devices.&#x20;

Enable password - Limits access to the privileged EXEC mode&#x20;

Enable secret - Encrypted, limits access to the privileged EXEC mode&#x20;

Console password - Limits device access using the console connection&#x20;

VTY password - Limits device access over Telnet

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