Network connection (III) functions and similarities and differences of hubs, switches and routers, routing tables and tables in switches, why do you need address translation and packet filtering?

3.0 Preface

• This chapter mainly talks about after the network package leaves the network card It is transmitted to the hub through the network cable , Then the hub pairs the packets Broadcast , The network packet is then sent to the switch , Switch According to the MAC Address Conduct Forwarding of network packets , Then the data arrives at the router , Router According to the IP Address forwarding , Then forward and deliver in a circular way , And many knowledge points involved in this process
  

3.1 Reach the hub through the network cable

• The structure diagram of the hub is as follows :

• The work of the hub can be put simply :

  • The network packet signal passes RJ-45 Transfer to hub , In the hub Relay circuit The signal will be processed to a certain extent Shaping and error suppression , After that, the network signal will be radio broadcast All ports of the hub

• Network cable passing " Twisted pair " To suppress noise
  

3.2 Through the switch

• The structure diagram of the switch is as follows :
  
• The work of the switch It can be put simply as :
  1. Packets arrive at the switch through the port , Switchboard Each port The structure of all similar a sheet No, MAC Address of the network card , It is the same as the network card ( See the last blog ) Send and receive data .
  2. After the switch receives the data , Query the address table inside the switch , See if it is in the network package MAC The record corresponding to the address , Find the output port
  3. Through the switching circuit The network packet is transferred to the corresponding output port , Then the output port forwards the data

• The address table inside the switch is the address table of the device MAC A record table of the address and which port the device is connected to is shown below :
  • Generally, the automatic maintenance of the switch , No manual maintenance required
    

3.3 Through the router

• The structure diagram of the router is shown below : It is divided into Forwarding module and Port module

• The working mode of the router is briefly described as :
  1. Receive the data packet through the port module , Then the forwarding module according to the IP The address is queried in the routing table , Further judge the next Forward destination address
  2. The forwarding module transfers the packet to the corresponding port ( The result of looking up the table )
  3. The port sends packets , Before sending the packet, it will first query according to the forwarding target address MAC Address , New use MAC Replace the old address MAC Address

• Structure of routing table
  

• The relationship between router and switch
  • In essence , Router and switch are entrusted , The router entrusts the switch to forward the network packet to the next router
  • Routers are based on IP Agreement work , Switches are Ethernet protocols that work

• The router address translation and Packet filtering function

3.4 The similarities and differences between switches and routers

Item	Router	Switch
Forwarding basis	According to the... In the head IP Address	According to the... In the head MAC Address
Look up the table matching method	Ignore the host number , Only match records with the same network number (IP Address )	Only match MAC A record with exactly the same address
" surface " Maintenance mode of	Packet forwarding and maintenance are independent of each other	The process of packet forwarding is automatically maintained , It is part of packet forwarding
When the packet does not know where to forward	Direct discarding / Forward to default route (0.0.0.0)	Broadcast to all ports