14.ip protocol -bite

IP Form of agreement

4 Bit version : 4/6(0100,0110), Main discussion IPv4.(0100)

4 Bit head length : and TCP similar .

TOS( Service type , Only 4 Bit effective , You can switch modes ): minimum delay , Maximum throughput , Highest reliability , Minimum cost , Only one form can be taken at a time
4.16 Bit total length : Single IP The maximum length of datagram cannot exceed 64KB(65536Byte), If you exceed ,IP The protocol itself implements such operations as subcontracting and grouping IP For packets , I don't care what is in the load at all. I just segment the data

. The following three fields are used for subcontracting and packaging
16 Bit identifier : The same IP Multiple datagram splits IP The identity of the datagram is the same
13 Bit shift : To describe the bag, who comes first and who comes later , Slice offset is not exactly a simple 1,2,3,4 Instead, you can calculate the space of the intermediate difference based on the slice offset . Put the last one in the middle IP Datagram insertion .( How to base on UDP Realize subcontracting ? The solution is to copy IP The homework )
3 Bit mark : Only one is good at ,0 It means there are follow-up ,1 It means the last package ( End mark )

8 Bit lifetime (TTL): It means a IP How many times can a datagram be forwarded over the network . When a datagram is sent , There will be an initial TTL( For example, common values ,128 perhaps 64)IP Datagrams go through one router at a time ,TTL It will decrease 1 If TTL Reduced to 0 了 , At this point, the router that receives the packet will discard the packet .
Some bags contain IP Address , May never arrive . A bag like this , It's impossible to forward endlessly on the network ( It takes up too much hardware resources )
natural IP Datagrams will be in the established TTL Arrive in .

8 Bit agreement : What kind of protocol does the transport layer use .TCP Or is it UDP Have different values

16 Bit header inspection and : It is correct enough to validate the data

32 Bit source IP Address : Source IP Indicates the sender's address
32 Purpose P Address : Purpose IP Indicates the recipient address .
about IPv4 Come on , One IP The address is essentially 32 An integer
You usually use “ dotted decimal " In this way to express this IP Address three points , hold 32 Bit integers are divided into 4 Parts of , Each part 1 Bytes . The value of each part is 0-255

IP Address management

IP An address is a dotted decimal data

IP The address is divided into two parts : network number + Host number
network number : Describe current network information ( Identification of LAN )
Host number : Distinguish the hosts inside the LAN

Be careful : In the same LAN , The network numbers between hosts are the same , The host number cannot be the same . Two adjacent LANs ( Connected to the same router ), The network number is also different .

distinguish IP Network number and host number in the address :
introduce Subnet mask : Subnet mask , Also a 32 position , An integer expressed in dotted decimal

Of the subnet mask It's all on the left 1, On the right 0( Can't 1,0 Mixed arrangement )
These on the left 1 It means which bits are network numbers
be left over 0 It means which bits are the host number

for example : In the general domestic scene , There are few LAN devices ( Not more than 255) The common subnet mask is 255.255.255.0
The decimal system is :
1111 1111 1111 1111 1111 1111 1111 0000 0000
In this LAN IP Address before 24 individual bit Time network number , after 8 individual bit Bit is the host number

If there are more LAN devices , Some other values will appear in the subnet mask

special IP Address :
If IP The host number of is all 0, The IP It means network number .( A normal device in the LAN , Host number cannot be set to 0)

If IP The host number of is all 1(255), The IP It means " Broadcast address ". Messages sent to this broadcast address , The whole LAN can receive

IP The address is 127 At the beginning , The IP All means " Loopback IP", Indicates that the host itself ,127.0.0.1( Loopback IP Typical representative of )
IP The address is 10 start ,192.168 start ,172.16-172.31 start , It means that we should IP The address is a LAN internal IP( Intranet IP)

besides , The rest IP be called Extranet IP( Used directly on the WAN IP)

Be careful : Extranet IP It must be unique . Every extranet IP Will correspond to a unique device .
Intranet IP It is only unique in the current LAN . In different LANs , You can have the same intranet IP The equipment

solve IPv4 The problem of insufficient address

1. Dynamic allocation IP Address : When the equipment is connected to the network, there are IP, Are you disconnected IP( There is no reduction in equipment ,IP The address has not been added : Treat the symptoms, not the root cause )

2.NAT Mechanism : Let multiple devices share the same external network IP.
Specifically :
NAT Mechanism , hold IP Divided into intranet ( LAN ) And the Internet ( Wide area network ), So the Internet IP It can be accessed anywhere on the Internet . Intranet IP It can only be accessed within the current LAN and is unique in the current LAN . At the same time, several devices in the intranet , You can share the same external network IP. here , Every extranet IP Can mean thousands of , Even tens of thousands of devices .

So when using the same extranet IP Multiple users access the extranet server through the LAN ( Extranet IP) when , The operator's router will be modified here IP The datagram , The source that will be sent from the intranet IP Change to the Internet IP, Router device with such function , Also known as NAT equipment .

Then through the ISP The same IP Multiple users of the same address access the same server at the same time ( Extranet IP) when , The port is used to distinguish , And return the respective response data

NAT Just a wave of renewal , But it doesn't solve the problem fundamentally

3.IPv6
IPv6 A longer field is used in the header to represent IP Address 16 Bytes ,128 position ,2 Of 128 Power

Each number is a hexadecimal number (4bit) Each colon is divided 2 Bytes

IPv6 It is said that it can distribute one to every sand on the earth IP Address

Routing

Routing , That is, planning the path .
Between two devices , To find a way , Can complete the process of transmission
To find the way , Premise is , You have to know the way first

IP The routing of the protocol is similar .IР Destination address in datagram , It means where the bag is going to be sent , This destination address , If the current router knows directly , Just tell you the way ~ If the current router does not know , Will tell you a general direction , Let you ask when you go to the next router
Go back in turn , In fact, it is getting closer and closer to the goal . At this time, you will always encounter a router that knows this address, so you can forward it concretely , sometimes , Not only did he encounter a router that knew the address, but he also knew several routes ~~ You can choose a more suitable way .