Reading notes of how the network is connected - understanding the basic concepts of the network (I)

《 How the network is connected 》 Reading notes - Understanding the basic concepts of network （ One ）

Talk about history

1991 year 8 month 6 Japan , At the center for nuclear research in Geneva, Switzerland （CERN） Working British physicist tim · berners · Li （Tim Berners-Lee）, Officially put forward World Wide Web, That is what we are very familiar with now www.

www What is it? ？ web WWW yes World Wide Web For short , Also known as Web、3W etc. .WWW It's based on the client / Server based information discovery technology and hypertext Integrated technology .

I'm curious here. We talk about it every day 3w,3w, however How the Internet came into being is not explained in the book ？

Here I checked the online information and explained a wave according to my personal understanding ：

In fact, the network first appeared in the United States during the cold war between the United States and the Soviet Union APRA Scientific research department , The scientific research department suddenly pulled out is in a mess and doesn't know what to do , No progress has been made in dealing with the scientific research work of the Soviet Union , Until a man named Robert · Tyler's buddies show up , His breakthrough was to find that small communication networks could not be compatible with different types of computers , We all know that the pinnacle of technology is making rules , There is no doubt that he chose to build a protocol so that all computers can work according to this set of rules .

So he found some big men and began to make trouble , In the middle, barabara did a lot of things and made many detours , The purpose is actually to achieve the above-mentioned things , Finally in a failed “LOGIN” Although only... Is transmitted during verification “LO” The two letters are disconnected , But this failure is a historic step forward , Because two different devices actually communicate , After the final repair, the normal transmission of these five letters has been completed .

Subsequently measured ARPANET（ ARPANET ） This project , That is, the formal prototype of the Internet .

This is the end of extracurricular knowledge , Now let's look at the main points in Chapter 1 ：

1. How to parse web address ？
2. DNS How does the server query the corresponding domain name IP Address ？
3. DNS How the server relays ？
4. How the browser delegates messages to the operating system and sends them to Web The server ？

The core is understanding DNS The role, status and role of , And how the browser follows DNS Interactive completion website （ domain name ） It can be interpreted as IP This is an operation , The delegation process at the end of this chapter is the key content of the second chapter , The notes are naturally placed in the notes in the second chapter , For the convenience of understanding, the notes are summarized in the second part .

How to parse web address

We from URL Start , What is? URL,URL yes Uniform Resource Locator For short , The professional explanation is called uniform resource locator , Apart from our common http、https Beyond the agreement , Browsers can also ftp Upload files , Download the file , Send email , Browse new culture and other operations .

We think of these behaviors as resource interactions , Although different resource interactions have different URL Combine , But no matter URL How does the combination form of change , In the end The beginning decides everything , The first part determines how resources are treated .

We can see the following example to parse the web address ：

img

The file name is omitted , There are usually the following ：

• http://xxxx/dir/ Express /dir/ The following content is omitted , At this time, the real access path corresponding to this directory is usually set for completion .
• about web The most classic http://localhost:8080/ Access path , Usually Web The server will access /index.html This file , If not, it will return 404 The page of .
• If only the domain name , such as www.baidu.com, Then you will directly visit web Resources and related files corresponding to the root path set by the server .
• Ambiguous paths such as http://localhost:8080/wishlist, The file name will be determined first , Then determine whether it is a directory , Or as a request mapped to another resource , Or do a redirect .

There is no need to delve into the above , Just understand The first step for browsers is to URL To analyze .

Http request

http The request can be summed up in one sentence ： What kind of operation has been done to what , The so-called "right" means URL, Represents the identified target object , What kind of operation to do is the so-called method , The method is mainly divided into two POST and GET Method , Other methods are basically useless , I have only encountered in a few docking documents PUT and HEAD Method .

img

GET Method ： It is usually used to access some visible resources , Or open access to resources , Usually, you can find the required resources directly from the specific directory without too many restrictions .

POST Method ： It is more common to use forms or AJAX Mode of access , And it usually points to a WEB Applications for , To obtain application data, you need to pass some valid parameters required by the server , Otherwise, the server will notify the client that it has no right to access according to the specific situation .

AJAX namely “Asynchronous JavaScript and XML”（ Asynchronous JavaScript And XML technology ）, Refers to a set of browser side web development technology that integrates a number of technologies .Ajax The concept of the concept by Jesse · James · Proposed by Jarrett [1].

Http The request message

got it What kind of operation has been done to what , Now let's see Http How did you do this .

Http The request message is mainly divided into the following organizational structure ：

• The first part of the first line is extracted URL The content of , Analyze it as it is , At the end of it is HTTP The version number mainly marks the current HTTP Request version .

for example ：GET /cgi/sample.cgi?Field1=ABCDEFG&SendButton=SEND HTTP/1.1

• The second line is the message header , Here are some simple things ：
  • Data： Request response generation date .
  • Pragma： Whether data is allowed to be cached .
  • Transfer-Encoding： Message body Coding format （ important ）.
  • Via： Through the proxy and gateway .
• There is a line after the message header Perfect blank lines without content .
• The fourth line is the message body , But the experiment used GET Method, so the content is usually empty .

img

Let's take Google as an example , The following is a request for reference to visit the Google search page , The content here is directly through Google browser F12 Copy , You can see that the request line is basically included , Message headers and message lines （GET Usually not, so the following is not reflected ） Three .

 routine 

1.   Request URL :https://www.google.com/
2.   Request method :GET
3.   The status code :200
4.   Remote address :127.0.0.1:7890
5.   Referral source website policy :origin

 Request header 

1.  :authority:www.google.com
2.  :method:GET
3.  :path:/
4.  :scheme:https
5.  accept:text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
6.  accept-encoding:gzip, deflate, br
7.  accept-language:zh,zh-TW;q=0.9,en-US;q=0.8,en;q=0.7,zh-CN;q=0.6
8.  cache-control:no-cache
9.  cookie:SID=KQi0QVpC_wxTynb6H6HjGmVq-9mYvCuIDOMx9EmEUJ8ii7dJzN_1F-ho69FdK6AN9ekOkA.; __Secure-1PSID=KQi0QVpC_wxTynb6H6HjGmVq-9mYvCuIDOMx9EmEUJ8ii7dJaTdIpqSfRfNb-BvF0haitA.; __Secure-3PSID=KQi0QVpC_wxTynb6H6HjGmVq-9mYvCuIDOMx9EmEUJ8ii7dJ6_WQQeEF09oAZ9MQfe21sA.; HSID=AOdmIhuBCutDeMwVS; APISID=ckyVXTB27QMaC2gQ/AVulr1cMnMbpD0e1x; SSID=AL0-0R0Ofsj3zaqrr; SAPISID=dqpTwJeh7bnii2Ki/AfsaDUfE8uMVR1aqv; __Secure-1PAPISID=dqpTwJeh7bnii2Ki/AfsaDUfE8uMVR1aqv; __Secure-3PAPISID=dqpTwJeh7bnii2Ki/AfsaDUfE8uMVR1aqv; SEARCH_SAMESITE=CgQIvJUB; 1P_JAR=2022-05-24-23; AEC=AakniGOKhznRpAD797X4u508i2XHJjEVYQQHANlqaJC2JSZ1F7mAe-vX_rg; NID=511=K-qt_LW-4ad1IYdJgfPLZjJw772wez2L3_FK9hwrrHAaksdhT8bTqz4icJEnJviOb92zcnyfS4h7P8HB_Is0f_FebYTe_5DR3qFEclHS1R9N1P7r9pv7Z4p12341S72RZRfzIlQ3-CVZUqQKBm1Xy1i9fKwejMGHTPMY2hk02sA--ey8nAEyt1_A7SVMe0RvrEkPnVm88fBnyyyFMMSCeSG1oqYKeC2x7iHJ0GwdbEpeGojpMQyQxAn1jAdxyXbC0oko0rCFjYn7eUREz2A9KA; SIDCC=AJi4QfGQeW0y_3pnzuBs7KI-WabF5XR_-dQchpcoNUN_bRVICBknb39qNQhP4IklnPn6kW4M3d8; __Secure-3PSIDCC=AJi4QfFOaoqiWv0mqmOskkIKVYy_-QNOATkPOyhNt9B8BBTMnRqnv-0zdgVgBNmIJRwlzBS4x6U
10.  pragma:no-cache
11.  sec-ch-dpr:2
12.  sec-ch-ua:" Not A;Brand";v="99", "Chromium";v="101", "Google Chrome";v="101"
13.  sec-ch-ua-arch:"arm"
14.  sec-ch-ua-bitness:"64"
15.  sec-ch-ua-full-version:"101.0.4951.64"
16.  sec-ch-ua-full-version-list:" Not A;Brand";v="99.0.0.0", "Chromium";v="101.0.4951.64", "Google Chrome";v="101.0.4951.64"
17.  sec-ch-ua-mobile:?0
18.  sec-ch-ua-model:""
19.  sec-ch-ua-platform:"macOS"
20.  sec-ch-ua-platform-version:"12.3.1"
21.  sec-ch-ua-wow64:?0
22.  sec-ch-viewport-width:1440
23.  sec-fetch-dest:document
24.  sec-fetch-mode:navigate
25.  sec-fetch-site:same-origin
26.  sec-fetch-user:?1
27.  upgrade-insecure-requests:1
28.  user-agent:Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/101.0.4951.64 Safari/537.36
29.  x-client-data:CLG1yQEIkrbJAQijtskBCMS2yQEIqZ3KAQjYjMsBCJShywEI2+/LAQjmhMwBCNupzAEI/qrMAQjDrMwBCKSvzAEYqKnKARirqcoB
30.  Decoded ：message ClientVariations { // Active client experiment variation IDs. repeated int32 variation_id = [3300017, 3300114, 3300131, 3300164, 3313321, 3327576, 3330196, 3340251, 3342950, 3347675, 3347838, 3348035, 3348388]; // Active client experiment variation IDs that trigger server-side behavior. repeated int32 trigger_variation_id = [3314856, 3314859]; }

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Response content

The response page is similar to the content information required to respond to the request line , At the same time, tell yourself what kind of request you are allowed to accept , If target IP Not found usually becomes 404 result .

 Response headers 

1.  accept-ch: Sec-CH-Viewport-Width  
2.  accept-ch: Sec-CH-Viewport-Height
3.  accept-ch: Sec-CH-DPR
4.  accept-ch: Sec-CH-UA-Platform
5.  accept-ch: Sec-CH-UA-Platform-Version
6.  accept-ch: Sec-CH-UA-Full-Version
7.  accept-ch: Sec-CH-UA-Arch
8.  accept-ch: Sec-CH-UA-Model
9.  accept-ch: Sec-CH-UA-Bitness
10.  accept-ch: Sec-CH-UA-Full-Version-List
11.  accept-ch: Sec-CH-UA-WoW64
12.  alt-svc: h3=":443"; ma=2592000,h3-29=":443"; ma=2592000,h3-Q050=":443"; ma=2592000,h3-Q046=":443"; ma=2592000,h3-Q043=":443"; ma=2592000,quic=":443"; ma=2592000; v="46,43"
13.  bfcache-opt-in: unload
14.  cache-control: private, max-age=0
15.  content-encoding: br
16.  content-length: 43734
17.  content-type: text/html; charset=UTF-8
18.  date: Tue, 24 May 2022 23:24:59 GMT
19.  expires: -1
20.  server: gws
21.  set-cookie: 1P_JAR=2022-05-24-23; expires=Thu, 23-Jun-2022 23:24:59 GMT; path=/; domain=.google.com; Secure; SameSite=none
22.  set-cookie: AEC=; expires=Mon, 01-Jan-1990 00:00:00 GMT; path=/; domain=www.google.com
23.  set-cookie: AEC=; expires=Mon, 01-Jan-1990 00:00:00 GMT; path=/; domain=.www.google.com
24.  set-cookie: AEC=; expires=Mon, 01-Jan-1990 00:00:00 GMT; path=/; domain=google.com
25.  set-cookie: AEC=; expires=Mon, 01-Jan-1990 00:00:00 GMT; path=/; domain=.google.com
26.  set-cookie: SIDCC=AJi4QfEXTiPm1BcAx1gfQzXOs-hmdcHylOVoSbbpy8cUIlP7hNwwfnfo_E8ZdTY1JZli8AqYYWk; expires=Wed, 24-May-2023 23:24:59 GMT; path=/; domain=.google.com; priority=high
27.  set-cookie: __Secure-3PSIDCC=AJi4QfFdxOIbJrwDKltt2sBRVFIcLOCyqQmgTSfYjXTYwqbhh0GPLcR9cxsgyaIh1j_GITbGeHc; expires=Wed, 24-May-2023 23:24:59 GMT; path=/; domain=.google.com; Secure; HttpOnly; priority=high; SameSite=none
28.  strict-transport-security: max-age=31536000
29.  x-frame-options: SAMEORIGIN
30.  x-xss-protection: 0  

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The response content is relatively simple , The main focus is the status code and response phrase in the first line , Whether the request execution results in success or failure . The above interactive content needs attention Http The request strictly follows that one request corresponds to one response .

Be careful ：1 Only... Can be written in request messages 1 individual URI. If you need to get multiple files , Each file must be sent separately 1 The request .

“ How to parse web address ？” This is the end of the introduction to this problem , Let's move on to the next question "DNS How does the server query the corresponding domain name IP Address ？".

IP and DNS

First of all, let's see IP and DNS What is it , Analysis website （ domain name ） And sending requests and responses seems to be done by the browser , In fact, it does not have these functions , The browser actually needs to rely on the operating system to send and receive network requests , The modern network is basically TCP/IP Of the world , therefore IP Play a key role , and DNS Storage IP And domain name mapping warehouse .

• IP： It can be compared to the specific position of our reality , such as XX road XX Number ,XX road （ network number ） The corresponding is assigned to The whole subnet Number , and XX Number （IP Address ） The number of is assigned to computers in the subnet , To obtain IP After the address, it is equivalent to locking the specific location of the current computer , Of course you can find it .
• DNS： Frankly speaking, it stores the domain name and IP The site of the mapped record , The browser needs to find DNS To know , Because the domain name is freely defined by the user, there will be multiple mappings of the same domain name IP.

Let's first observe XX road XX The number of the number is IP How the address was found .

The sending network must first pass through the hub through the subnet , Forward the content to the nearest router , Then the router will continue to cross to find the next router closer to the recipient , Finally, find the hub and forward it to the receiver's router , The router is the role of the steering wheel here , and DNS Addressing is like navigation .

img

What are hubs and routers ？A　 Router ： A device that forwards packets .B　 A hub ： A device that forwards packets , It is divided into relay hub and switching hub .

IP The composition of the address

IP The address is a 32 Bit Bit Count , adopt 8 individual bit Divide into... As a group 4 Group , Each group is represented in decimal and broken by small dots .

But this alone 32 individual bit Is unable to locate IP Address and network number , So we need to give IP Address additional information .

Usually IP There are the following ways to combine addresses ：

a）IP Address body ：11.22.33.44

b）IP And subnet mask ：11.22.33.44/255.255.255.0

c） The number of bits of the network number is represented as a subnet mask ：11.22.33.44/24

d） Subnet address ：11.22.33.0/24, This represents the entire subnet

e） Subnet broadcast address ：11.22.33.255/24

First of all, the whole thing IP It is divided into 32 Bit fixed length , Each group of four is separated by dots , adopt 0 - 255 The decimal representation of , such as 0 Namely 8 individual 0,255 Namely 8 individual 1, But just by 32 The single digit is Can't tell this 32 The bit part is the network number , That part is the host number , So here we are IP Basic composition of ：

• 11.22.33.44： Indicates basic IP.
• 11.22.33.44/255.255.255.0： The first part is basic IP, The following section represents the subnet mask , Expression and IP The address is consistent , Pay attention to the last 8 All bits are 0 Represents the entire subnet .
• 11.22.33.44/24： This 24 It's also a subnet mask , But the way to represent a subnet is to use the number of bits .
• 11.22.33.0/24： The subnet mask has not changed ,IP The address has changed , final 8 position 0 It also represents the whole subnet .
• 11.22.33.255/24： The subnet mask has not changed ,IP The address has changed , final 8 All parts 1 Express radio broadcast The whole subnet .

What is a subnet mask ？

Concepts that may be difficult for beginners to understand , If you don't understand it here, it is recommended to find more information and choose a understandable explanation （ The key is to understand and repeat ）.

Here is a picture from the book , I don't feel very intuitive after reading it and I don't know what to do , So I made a supplementary explanation .

In fact, the network number in the following figure is AND（ Bit operation &） Calculate the network number , After calculation, confirm that the network number is 10.1.2, And the part omitted later is actually 8 individual 0 Mask to mask the host , The subnet here is 10.1.2.0, Corresponding host IP yes 10.1.2.3.

If you don't understand what it says , Here is a supplementary explanation ：

First of all, we need to know what the subnet mask is for , To put it bluntly, it is to divide the network number and the host number , The network number can be regarded as a building , The mainframe is a small room inside , But the room number may be 101, It could be 0101, More likely 00101, It depends on the room number covered 0000 To 1111 How many , Decide how many hosts can be allocated , And computers 0 And very special , So the part of the network number + The bit part masked by the subnet mask （ Performance is all 0） Form a subnet , Only hosts on the same subnet can communicate with each other .

therefore The subnet mask cannot exist alone , It has to be combined with IP Address used together , The subnet mask is mainly divided into two parts ： All for 1 Indicates the network number , All for 0 Indicates the host number .

Then we can set a subnet mask randomly, such as 255.255.255.111 OK? ？

The answer is no way , no way , no way . because Subnet mask Must be Successive 1 and 0 form , The mask above 255.255.255.0 It's the right way to use , Another example is 255.255.255.244 It is also a qualified subnet mask .

Let's give an example of ：

1100 0000.1010 1000.0000 0001.1000 0001 -- 192.168.1.129
1111 1111.1111 1111.1111 1111.0000 0000 -- 255.255.255.0
———————————————————— 
1100 0000.1010 1000.0000 0001.0000 0000 --  Divide the final subnet segment ：192.168.1.0
 notes ： Confirm that the final network number and host network segment use bit operation  & （1 & 1 = 1,1 & 0 = 0）

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From the final result 192.168.1.0 In front of 24 All for 1 It's exposed , And the back 8 Bit is masked by subnet mask , So his other way of writing is 192.168.1.0/24, Express 24 Is it 1 Represents the network number , remainder 8 Bits are used to represent the host number, but because they are masked, they are all 0.

Although it is covered up , But the host number location is 0 yes 1 No need to care , If the host number falls in 192.168.1.0 To 192.168.1.255 Indicates that subnets in the same network segment are treated equally , such as 192.168.1.129 It means that 129 position , Included in this network segment .

Summary

1. Subnet mask Must be Successive 1 and 0 form , Simple memory is 1 To divide the network number , by 0 Is used to mask the host number , Hosts in the same subnet can communicate with each other .
2. There are two ways to write subnet masks. One is to use and I P The address is the same 32 Bit full representation , The other is to use numerical calculation 1 Number of bits , such as 24 It is in the following figure 24 individual 1.
3. IP Host number of the address , whole 0 Represents the entire subnet , whole 1 Means to send packets to all devices on the subnet . Here I add that it is all 1 The situation of - “ radio broadcast ”.

What does broadcasting mean ？ Broadcast address is an address specially used to broadcast to all hosts in the network at the same time , Any host in the same network segment can listen to the radio .（ for example 192.168.211.32/24 The broadcast address of is ：192.168.211.255）

DNS

DNS：Domain Name System DNS , The most common use is to match the server name with IP Address , Of course, it is only its main business , He has some sidelines that are very important but not the key. Just be interested in learning about them .

IP It's hard to remember , Short domain names are easier to remember . You may find it hard to remember a long domain name , Although the analysis IP Is much faster than resolving domain names , But it is obviously impossible for netizens to remember a string of numbers in order to visit a website , At the same time, the domain name was also meaningful for the development of the Internet at that time .

So who will tell the truth of the host domain name IP How much is the ？ Now DNS On the stage ,DNS Is used to map IP And domain name .

DNS How to complete the mapping ？ For the user host, there must be one for parsing DNS The client of , This client is often referred to as a parser , Resolve through the domain name IP The process of addressing is also known as Domain name resolution .

Call the parser

So how to call the parser ？ A parser is actually a piece of program code , This piece of program code is contained in the... Of the underlying operating system Socket In the library , adopt Socket Library call parser , Then the parser will send DNS The server sends a query message , DNS clothing Server queries according to the request IP Then return the response message .

Socket The process of resolving domain names is very simple , It only takes one line of code to complete ：gethostbyname("www.lab.glasscom.com");.

library ： Refers to a collection of general program components , It is used to standardize the construction of program code . Take the parser as an example ,Socket A library is a collection of program components used to call network functions .

How does the parser work internally ？

When the application calls the parser , At this point, control is transferred to the parser , When the control is transferred to the parser, the parser will generate a message to send to DNS Query message from the server , The generation process is similar to building a “ Please let me know XXX Of IP Address ” The news of , And send it to DNS The server finishes parsing .

Pay special attention to Sending the request is not done by the parser , It needs to be delegated to the operating system again Protocol stack complete , So the permission will be transferred to the protocol stack again , The protocol stack finally sends the message to through the network card DNS, then DNS find out IP Return message , This time DNS Parsing the request is complete .

It is worth noting that The parser will take out IP Address is written to the application Specified memory address in , Also note the request DNS The server itself IP It also needs to be configured , It's just this IP The address is for TCP/IP The project has been set up in advance , You don't need to query any more to get .

Finally, it can be obtained in different operating systems DNS There will be differences in the address mode of the server .

According to the description above , summary DNS Analytical process ：

1. WEB The browser sends a domain name resolution request , adopt Socket Request parsing to the parser , At this point the application will hang .
2. The parser is responsible for “ translate ” Application's DNS Resolve request , But delegate specific request operations to the system stack .
3. The system stack is responsible for sending the request to through the network card DNS The server , wait for DNS Feedback results .
4. DNS Get real IP The result is then sent back to the application through the link .

DNS Work

Now let's see dns How to work ,dns The user message to be queried includes the following parts ：

• domain name ： To replace IP Convenient memory .
• class： Considering the situation outside the Internet , Of course, the external network is only marked as in Internet .
• type： Indicates that the record type is used to distinguish dns The way of parsing , Different types have different results .

It should be noted that all a Types of records are recorded in DNS Registered. , Another example is that all mail types are MX The type of , Whole DNS The job is to simply look up the table according to the type and domain name , Find a match and send it back , Otherwise you won't find .

Domain name hierarchy ： The so-called domain name hierarchy is DNS It can't be a server , It requires the cooperation of multiple servers , Each manages its own scope of content , For the domain name, the part on the right is higher in the domain name hierarchy .

Is that interesting , Taking everyday life as an example is not what we think www It's the highest , In fact, he is the lowest grade ,.com Is the highest .

DNS Find steps

First, I will be responsible for the management of the subordinate domains DNS Server's IP The address is registered with their superior DNS Server , Then the superior DNS Server's IP Register the address to a higher level DNS Server , And so on .

for instance , The domain name that needs to be resolved is www.baidu.com,DNS You need to put the whole www.baidu.com The entire domain name is registered to baidu.com, And then baidu.com Sign up to com domain name , And then com Register to the root domain , In this way, you can find your subordinates through your superiors .

Mention here DNS There is a root domain , The root domain name refers to a period . , and Linux The root path of is similar in meaning and form .

The root domain can explain why any strange domain name can be accessed , The reason is that all subordinates will be registered with their superiors , Finally, there is a common , If all subordinates want to find the root domain , Naturally, you need to register the root domain .

DNS The process of searching is to find the nearest one from the bottom up DNS Then find the root domain from top to bottom , That is to find Current dns look for , If it is not found, you need to find it directly from the root domain , All the way down the hierarchy to find the final ip, That's why visiting foreign countries ip The reason for the slow , Because the routing link is really long , It may take dozens of times dns Search for .

DNS cache

If every host always passes every request DNS Get the... Corresponding to the domain name IP Information , It is impossible to always DNS Let's find it again , In fact, it can be parsed into reality IP It will be cached in advance, and the repeated data accessed next time can be returned directly .

In addition, although the previous introduction is to search from top to bottom from the root domain , actually DNS You can also use the cache feature to speed up access through multiple levels of shared domain names and cross level search , such as www.baidu.com and www.baidu You can quickly find the shared cache by skipping two levels （ Of course, here is the nearest one DNS You can find , Here are just examples ）.

But caching has a side effect , It just doesn't exist ip Address messages are also cached , However, in order to prevent this situation, there is usually an expiration time , After the expiration, you still need to walk again DNS Find process .

Problem guidance ： problem

• http://www.nikkeibp.co.jp/ Medium http What does it mean ？

It means HTTP agreement

• What are the differences between the following two web sites ？

a. http://www.nikkeibp.co.jp/sample b. http://www.nikkeibp.co.jp/sample/ The difference is sample It may be resolved that the result does not conform to the expectation . It is used to identify the addresses of computers and servers connected to the Internet ？ IP Address according to Web The domain name of the server IP What is the name of the server used for the address ？ DNHS The server towards DNS What is the name of the program that the server sends the request message ？ Parser .