127.0.0.1, 0.0.0.0 and localhost

Reproduced in ：https://zhuanlan.zhihu.com/p/439510282

Let's take a look at what we have in common ：

• All belong to special address .
• All belong to A Class address .
• All are IPV4 Address .

Next, let's look at the three addresses respectively ：

0.0.0.0

First ,0.0.0.0 Can't be ping common .0.0.0.0 be called “unspecified”, I.e. not specified （ Is invalid , Meaningless ） Address . functionally , Generally used in some programs / It is inconvenient to use in the network protocol ip In special circumstances （ To put it bluntly, it is a case for some comparison pits “ Place holder ”）, such as DHCP The client has not obtained ip It is stipulated to use 0.0.0.0 do “ source address ”, Or in the server ,0.0.0.0 Not a real IP Address , It represents all... In this machine IPV4 Address . When the server does not specify which network card to listen on , Also used 0.0.0.0, At this time, listen to all in this machine IP The port of .

purpose

• Used as a server , Any on this machine IPV4 Address .

127.0.0.1

First of all, we need to know a concept , All with 127 At the beginning IP Address , It's all loopback addresses （Loop back address）, The loopback interface is generally understood as a virtual network card , Not a real router interface .

The so-called loopback address , Generally speaking , That is, we send it to... On the host 127 At the beginning IP The address packet will be received by the sending host itself , It doesn't get out at all , The external device cannot access the local device through the loopback address .

Last 127.0.0.1 Belong to {127,} One of the sets , And all network numbers are 127 The addresses of are called loopback addresses , So the return address ！=127.0.0.1, They are containment relationships , That is, the loopback address contains 127.0.0.1.

purpose

• Loop back test , By using ping 127.0.0.1 Test a network device on a machine , Operating system or TCP/IP Whether the implementation works normally .
• DDos Attack defense ： Website received DDos After the attack , Domain name A It was recorded that 127.0.0.1, Let the attacker attack himself .
• Most of the Web The native address of the binding when the container is tested .

localhost

comparison 127.0.0.1,localhost Have more meaning .localhost It's a domain name , Instead of a ip Address . The reason why we often put localhost And 127.0.0.1 I think it's the same because most of the computers we use say localhost Yes 127.0.0.1 This address .

however localhost The meaning of is not limited to 127.0.0.1.

localhost Is a domain name , Used to refer to this computer perhaps this host, It can be used to obtain network services running on the local machine .

In most systems ,localhost Was pointed to IPV4 Of 127.0.0.1 and IPV6 Of ::1.

127.0.0.1 localhost
​
::1 localhost

therefore , Pay attention to confirm when using IPV4 still IPV6

summary

127.0.0.1 It's a loopback address . It doesn't mean “ This machine ”.0.0.0.0 That's what it really means “ Local machine in this network ”.

in application , In general, when Binding ports on the server, we can choose to bind to 0.0.0.0, So that my service visitors can go through my multiple ip Address access to my services .

For example, I have a server , An external address A, An intranet address B, If the port I bind specifies 0.0.0.0, Then I can access my application through intranet address or Internet address . But if I bind an intranet address , Then you can't access it through the Internet address . So if binding 0.0.0.0, There are also some hidden dangers , For services that only need intranet access , You can only bind the intranet address .

(127.0.0.1 Only to the kernel protocol stack ,0000 I have to go to the network card , The network card is interrupted )