CCNA basics

CCNA basics


CCNA (Cisco Certified Network Associate) is a network technology assistant certification, which is Cisco's primary certification.

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The four elements of the network

  1. Rules: Rules or agreements govern how messages are sent, directed, accepted and interpreted
  2. Message: The transmission of a message or unit of information from one device to another
  3. Medium: A medium is a tool that connects these devices, that is, a medium that can transmit messages from one device to another
  4. Devices: Devices on the network exchange information with each other
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Network topology

Computer network topology is a method of referencing the relationship between points and lines that have nothing to do with the size and shape of the topology. Computers and communication devices in the network are abstracted as a point.

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Bus topology

The bus topology is to directly connect all the devices in the network to the public bus through the corresponding hardware interface, and the nodes communicate in a broadcast model. The information sent by a node can be "listened" to other nodes on the bus.

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Pros and cons:

  • Advantages: simple structure, easy wiring, high reliability, easy to expand. It is a topological structure for short-range use in local area networks
  • Disadvantages: All data must be transmitted through the bus. The bus becomes the bottleneck of the entire network, and it is difficult to diagnose faults

Star topology

Star topology means that each node is connected to the central point through a separate communication line.

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Pros and cons:

  • Advantages: simple structure, easy to implement, easy to manage, easy to monitor and eliminate connection point failures
  • Disadvantages: The central node is a reliable bottleneck for the entire network. Once the central node sends out failures, the network will be paralyzed.

Ring topology

Ring topology means that each node forms a closed loop through the communication line, and the data in the loop can only be transmitted in one direction.

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Pros and cons:

  • Advantages: simple structure, easy to implement, suitable for optical fiber, long transmission distance, definite transmission delay
  • Disadvantages: Each node in the ring network is the bottleneck of the network reliability of the Urban Management Committee. Any node failure will cause the network to be paralyzed, and the fault diagnosis is also very difficult

Tree topology

A tree topology is a hierarchical structure in which nodes are connected by layers. Information exchange is mainly carried out between upper and lower nodes. Generally, no data exchange is carried out between adjacent nodes or nodes of the same level.

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Pros and cons:

  • Advantages: simple connection, convenient maintenance, suitable for mobile phone information application requirements
  • Disadvantages: low resource sharing ability and low reliability. Any workstation or connection failure will affect the operation of the entire network

Mesh topology

Network topology is also called irregular structure, and the connections between nodes are arbitrary and irregular.

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Pros and cons:

  • Advantages: The system has high reliability and is relatively easy to expand.
  • Disadvantages: complex structure. Each node is connected to multiple points, so routing algorithms and flow control methods must be used

Protocol and standardization

Standardization Organization

Institute of Electrical and Electronic Engineers: IEEE (Institute of Electrical and Electronics Engineers)

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International Organization for Standardization: ISO (International Organization for Standardization)

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Electronic Industries Association: EIA (Electronic Industries Association)

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OSI model

In 1971, the ISO organization could not publish the OSI reference model. The communication system interconnection reference model developed by OSI (Open System Internetwork).

OSI seven-layer model:

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Physical layer

Physical layer (Physical): Binary transmission:

  • Defines electrical specifications, mechanical specifications, process specifications and functional specifications for starting, maintaining and closing physical links

Data Link: Access to the medium:

  • Define how to format data for transmission and how to control access to the network
  • Support misalignment detection

Network layer

Network layer (Net Work): Data transmission:

  • Routing packets
  • Choose the best path to transfer data
  • Support logical addressing and path selection

Transport layer

Transport layer (Transport): Transmission problems:

  • Ensure the reliability of data transmission
  • Establish, maintain and terminate virtual circuits
  • Through error detection and recovery
  • Information flow control to ensure reliability

Session layer

Session: Communication between hosts:

  • Establish and terminate sessions between applications

Presentation layer

Presentation: Data representation:

  • Ensure that the receiving system can read the data
  • Format data
  • Build the data
  • Negotiate the data transmission syntax for the application layer
  • Provide encryption

Application layer

Application layer (Application): network short-range access application layer:

  • Kick the dog network service for the application process
  • Provide user authentication

to sum up

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TCP/IP model

Application layerProvide users with data plus coding and dialogue control
Transport layerSupport communication between different devices through different networks
Internet layerDetermine the best path through the network
Network access layerControl the hardware devices and media that make up the network
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Network foundation

Communication mode

Data is usually transmitted between two stations (point-to-point). According to the direction of the data flow, it can be divided into three transmission modes:

  • Simplex
  • Half duplex
  • Full duplex


Simplex communication only supports signal transmission in one direction, and the transmission direction of the model cannot be changed at any time.

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Pros and cons:

  • Advantages: simple equipment, power saving, low cost
  • Disadvantages: Due to the use of PPT (Push-to-talk), it is not convenient to use

Half duplex

Half duplex (Half Duplex) allows signals to be transmitted in two directions, but only allows one-way transmission of signals on one channel at a time. (such as walkie-talkies)

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Pros and cons:

  • Advantages: simple equipment, power saving, low price
  • Disadvantages: any need to press the button to speak, it is not convenient to use

Full duplex

Full duplex (Full Duplex) Both parties can transmit information at the same time. In full duplex mode, each end of the communication system is equipped with a transmitter and a receiver. Therefore, it is possible to control the data to be transmitted in both directions at the same time.

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Pros and cons:

  • Advantages: easy to use, no need to switch
  • Disadvantages: complex equipment, high power consumption

transfer method

There are three communication modes in the current network:

  • Unicast
  • broadcast
  • Multicast


Unicast refers to the reception and transmission of letters only between two nodes.

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Multicast can achieve the purpose of transmitting data to all target nodes or only transmitting data to specific objects.

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One pair of all communication modes between hosts, the network unconditionally copies and forwards the information sent by each host, and all hosts can receive all the information.

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Anycast (Anycast) is a routing method for network addressing. In general, there is a one-to-one relationship between an IP address and a server on the Internet. When a unicast address is assigned to multiple interfaces, it becomes anycast address.

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Conflict domain and broadcast domain

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Conflict domain

The Collision Domain is a collection of all nodes on a unified physical network segment. This domain represents the area in which collisions are sent and propagated.

Isolate the conflict domain:

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Broadcast domain

Broadcast Domain (Broadcast Domain) is a logical division of the computer. Any node in the broadcast can reach any node by broadcasting at the data link layer. The broadcast domain can be deployed in the same LAN or be bridged to other LANs .

Isolate the broadcast domain:

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Data header

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IP address

An IPv4 address mainly consists of two parts:

  • One part is used to identify the network number to which the address belongs
  • One part is used to identify a specific host in the network
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IP address range

IP address classThe first set of binary octets binaryThe first set of binary eight-digit decimal valuesNumber of possible hosts
Class A1-12600000001 to 01111110*16, 777, 214
Type B128-19110000000 to 1011111165, 534
Type C192-22311000000 to 11011111254

Note: 127 (01111111) is a class A address reserved for the loopback test and cannot be assigned to the network.

IP address structure

  1. Address category: Indicates which category the address belongs to
  2. Network bit: Indicates the network segment to which the address belongs
  3. Host bit: the unique identification of the device in the network segment

IP address type

  1. Network address: indicates the address of the network segment
  2. Broadcast address: an address that can reach everyone
  3. Host address: the unique address of each device in the network segment

Private address

classPrivate address range
A10.0.0.0 to
B172.16.0.0 to
C192.168.0.0 to

The port number

The role of the port number is to identify the services provided, and different services should use different ports. The default port can be changed, but the client must change it synchronously.

The transport layer protocol uses port numbers to mark and distinguish various upper-layer applications. As shown in the figure:

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Subnet mask

The subnet mask can help us distinguish between network identification and host identification, and divide a large IP network into several small subnets.

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Principle of subnetting:

  • After the IP address is divided into a subnet, it is divided into network bits, subnet bits and host bits.
  • Number of subnets = 2^n, where n is the number of bits in the subnet
  • Number of hosts = 2^n-2, where n is the number of bits in the host
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