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Star structure is a center , Multiple sub nodes . It has a simple structure , Easy to connect , Management and maintenance are relatively easy , And it's scalable . The network latency is small , Low transmission error . There is no fault in the center , Generally, there is no problem with the network . Central failure , The network goes wrong , At the same time, the sharing ability is poor , The utilization rate of communication line is not high .

Bus topology

Bus topology all devices are connected to one connecting medium . The bus structure requires a small number of cables , The cable length is short , Easy wiring and maintenance . Multiple nodes share a transmission channel , High channel utilization . But don't find diagnostic trouble .

Ring topology network

A ring topology network is one in which nodes form a closed loop . Fewer workstations , Saving equipment . Of course , This leads to a node problem , The network will go wrong , And it's hard to diagnose the fault .

Tree topology

Tree topology evolved from bus topology , Shaped like an inverted tree , The top is the root , There are branches below the root , Each branch can also be branched , The tree root receives the data sent by each site , Then broadcast to the whole network . Good expansion , Easy to diagnose errors , But the requirements for the root are high .

Network topology

The most widely used , Its advantage is that it is not affected by bottleneck problems and failure problems , One day the line went wrong , Can do other lines , But it's too complicated , The high cost .

The role of the switch

1、 Provide port density （ Inherited HUB The role of ）

2、 Based on data identification and retransmission , It realizes theoretically infinite transmission distance

3、 Based on data recognition 、 Store and forward , Solved the problem of conflict

4、 be based on MAC Address recognition 、 Record 、 Inquire about , Realize unicast communication

How the switch works

The switch works in the media access control layer , Identify and convert current to binary , After the data current enters the switch , The switch first recognizes it as a layer-2 binary , Then identify the source in the data frame mac Address , Record the local MAC In the address table ,（MAC Record each MAC Corresponding interface ）, Then focus on the target in the data frame MAC Address , Then query the local MAC Is there any record in the table , If there are records , Only record based interface unique forwarding （ unicast ）, If there is no record, flooding will occur .

notes ： Flooding means that in addition to the access interface of flow , All other interfaces copy out .

What routers do

1、 Interconnection between different networks

2、 Cut off the broadcast domain

3、 Choose the path for the data it carries --- Route selection

How routers work

When the packet enters the router , First check the target IP Address , Then query the local routing table , If there are records in the table , take Unconditional Forward as recorded , If there is no record , The traffic will be discarded .

OSI Seven layer reference model

application layer ： Input and output of abstract language , Encode abstract language .

The presentation layer ： Convert the encoding to binary ( code decode encryption Decrypt )

The session layer ： Application internal address , Distinguish between sessions in the program Find out -- establish -- maintain -- Terminate the conversation

Transport layer ：1. Provide port numbers to distinguish different services （ Static port number 1---1023 Dynamic port number 1024---65535 Special port number 0--- Represents all port numbers --- Network programming uses ） 2. Data segmentation （ suffer MTU Limit ）3. Provide reliable transmission --TCP/UDP

The network layer ：Internet agreement --IP （ Addressing Addressing according to IP Address for logical addressing ）

Data link layer ： according to MAC Address for physical addressing （LLC Logical link control layer MAC Media access control layer ）

The physical layer ： Define the electrical voltage 、 Optical characteristics , The interface specification

TCP/IP agreement

TCP/IP The agreement refers to OSI Architecture of .OSI The model has seven layers , But it's more complicated , So in TCP/IP Agreement , They are reduced to four levels

（1） application layer 、 The presentation layer 、 The services provided by the three layers of conversation layer are not very different , So in TCP/IP Agreement , They are merged into a hierarchy of application layers
（2） Because the transport layer and network layer are very important in the network protocol , So in TCP/IP In the protocol, they are regarded as two independent levels
（3） Because the contents of the data link layer and the physical layer are not much different , So in TCP/IP In the protocol, they are merged in a layer of the network interface layer .

Encapsulation of data

application layer ： Raw data is converted into binary data
Transport layer ： Binary data is divided into small data segments , And package TCP Head （ Data segment ）
（TCP The key information of the head – Port number ）
The network layer ： The data from the transport layer is encapsulated IP Head （ Data packets ）
（IP The key information of the head –IP Address ）
Data link layer ： The data from the network layer is encapsulated MAC Head （ Data frame ）
（MAC The key information of the head –MAC Address ）
The physical layer ： The bit stream composed of binary data is transformed into electrical signal and transmitted in the network （ Bit stream ）

TCP

TCP： Transmission control protocol -- Connection oriented reliable transport protocol

After completing the basic work of the transport layer , It also needs to further ensure the reliability of transmission

Connection oriented ： adopt TCP Three handshakes of establish an end-to-end virtual link

Reliable transmission ：4 A reliable mechanism -- confirm 、 Retransmission 、 Sort 、 Flow control （ The sliding window ）

UDP

UDP: user datagram protocol -- Non connection oriented unreliable transport protocol

Only the basic work of transmission is completed --- piecewise 、 Port number

URL

URL（Uniform Resource Locator） Uniform resource locator , It's the computer Web Network related terms , It means web address . Our Internet world is made up of many URL form , It can also be said that URL To show .

URL It's made up of three parts ： The resource type 、 Host domain name where resources are stored 、 Resource file name .

DNS

DNS It's the domain name system （Domain Name System） For short , On the Internet as domain names and IP A distributed database with address mapping , It can make it easier for users to access the Internet , Instead of remembering what can be read directly by the machine IP Address .

Domain name resolution process ：
(1) After entering the domain name , First find the domain name server corresponding to your host , The domain name server first looks up the data in its own database .
(2) without , Search the superior domain name server , By analogy
(3) Up to the root domain name server , I'm sure I can find this domain name IP Address
(4) The domain name server itself does some caching , Compare the domain name you have visited with the corresponding IP The address is cached , Can speed up the search process

IPv4 Address

IPv4 Address by 32 Bit binary structure , Expressed in dotted decimal , With global uniqueness （ Public address ）

Public address ：

A class ---C class

A:8 Bit network bit ,24 Bit host bit composition .0.0.0.0-127.255.255.255. 0-127

B:16 Bit network bit ,16 Host location . 128.0.0.0-191.255.255.255

C:24 Bit network bit ,8 Host location .192.0.0.0-223.255.255.255

D Class or E class :
D: Multicast address . 224.0.0.0-239.255.255.255.

E: Reserved address , Not yet used . 240.0.0.0-255.255.255.255

Private address ：10.0.0.0-10.255.255.255 172.16.0.0-172.31.255.255 192.168.0.0-192.168.255.255

VLSM： By extending the length of the subnet mask , From the original host bit to the network bit , Realize the segmentation of a network number into multiple . Every new subnet , There will be fewer hosts , Add network number , Reduce the number of users in each network number , Increase the number of network segments

CIDR： No inter domain routing -- Take phase parity , Go to different places , Logically synthesize multiple network numbers into one

Subnet summary -- After summary , The mask of the summary network segment is longer than the main class
HYPERNET -- After summary , The mask of the summary network segment is shorter than the main class mask

Static routing

Static routing , It is the route added by the network administrator for the router by manual configuration . The popular saying is ： The network administrator tells the router through manual configuration ：“ You want to reach your destination X, The data packet needs to be transferred from the interface Y Throw it out for the next jump Z.

[r1]ip route-static 192.168.3.0 24 192.168.2.2
（ Target segment ）（ Next jump ）

Loopback interface

stay TCP/IP The middle loop device is a virtual network interface realized by software , It is not associated with any hardware . meanwhile , It can be used in experimental environment , Analog connection PC User interface of terminal , To reduce the actual equipment cost demand .

[r1]interface LoopBack ?
<0-1023> LoopBack interface number（ Available range of loopback interface ）
[r1]interface LoopBack 0
[r1-LoopBack0]ip address 1.1.1.1 24

Default route

A route that does not limit the destination , Represents all network segments , Router When looking up the table, you are querying all local direct connections 、 static state 、 If there is still no reachable path after dynamic routing , Use this entry .

[r1]ip route-static 0.0.0.0 0.0.0.0 192.168.1.1
（ Next jump ）

Floating static

A special static routing , Go to the same destination network segment through configuration , But static routes with different priorities , In order to ensure a higher priority route in the network , That is, when the main route fails , Provide backup routing . Under normal circumstances , The backup route will not appear in the route table .

[r1]ip route-static 100.100.100.0 24 13.1.1.2 preference ?
INTEGER<1-255> Preference value range（ Priority range ）
[r1]ip route-static 100.100.100.0 24 13.1.1.2 preference 61

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