Network layer: IP protocol

ip Protocol related concepts

• host : Equipped with ip Address , But the device that does not perform route control !
• Router : Equipped with ip Address , And can do their best to control the route !
• node : Host and router !

Form of agreement

• 4 Bit version number (version): instructions ip Version of protocol , about IPv4 Namely 4,IPv6 Namely 6!
• 4 Bit head length (header length) : IP The length of the head is 32bit, That is to say length*4 Bytes of ,4bit The maximum length of the representation of is 15, therefore ip The maximum length of the head is 60 byte !
• 8 Bit service type (Type Of Service): 3 Bit priority field ( Have been abandoned ),4 position TOS Field , and 1 Bit reserved field ( Must be set to 0).
• TOS: 4 Bits denote respectively minimum delay , Maximum throughput , The most reliable , Minimum cost . These four conflict with each other , Only one... Can be chosen !
• 16 Bit total length : IP How many bytes does the datagram occupy as a whole !
• 16 Bit mark (id): It uniquely identifies the message sent by the host . If IP Messages are fragmented at the data link layer , So every piece of id It's the same !
• 3 Bit flag field : The first one is reserved , The second is 1 Indicates that fragmentation is prohibited ( At this time, if the message length exceeds MTU ,IP The module will discard the message ), The third means " More pieces ", If IP The message is fragmented , The third position mark of the last partition is 1 End of expression , Others are set to 0!
• 13 Bit slice offset (frameagement offset): It's the shards relative to the original IP Offset at the beginning of the message .
• 8 Bit lifetime (Time to Live TTL): The maximum number of message hops when datagram arrives at destination . The initial value is 64, Every time we go through a route ,TTL -= 1, Down to 0 It hasn't arrived yet , Then discard it . This field is mainly used to prevent routing loops !
• 8 Bit agreement : Indicates the type of upper layer protocol !
• 16 Bit header checksum : Use CRC Painstaking verification , To identify if the head is damaged !
• 32 Bit source address ,32 Bit destination address : Indicates the sender and receiver !

Segment division

IP The address is divided into two parts , Network number and host number

• network number : Ensure that two interconnected network segments have different identifications !
• Host number : Ensure that in the same network segment , Hosts have the same network number , But the unused host must have a different host number !
• Different subnets put together hosts with the same network number !
• If you add a host to the subnet , Then the network number of this host is consistent with that of this subnet , But the host number cannot be the same as other hosts in the subnet !
• DHCP : It can assign to new hosts in the subnet IP Address , Avoid the trouble of manual management !
• Subnet mask : Used to distinguish network number and host number , Usually 32 Positive integer , take IP The address and subnet mask are bitwise AND , The result is the network number !
• special IP :
  • take IP All host addresses in the address are set to 0, It becomes a network number , Represents a local area network !
  • take IP All host addresses in the address are set to 1, It becomes a broadcast address , It is used to send datagrams to all hosts connected to each other in the same link !
  • 127.* Of IP Address is used for local loopback test , Commonly used 127.0.0.1!

private IP and Public network IP

If a local area network is set up within an organization ,IP Address is only used for LAN communication , Instead of connecting directly to Internet On , Use any IP The address can be , however RFC1918 It specifies the private use of the local king of components IP Address :

• 10.* The first eight digits are network numbers , common 167772176 An address !
• 172.16. To 172.31. front 12 Bit is the network number , common 1048576 An address !
• 192.168.* , front 16 Bit is the network number , common 655535 An address , Included in this scope , Are called private IP , The rest are called public networks IP( overall situation IP)!
• One router can be configured with two IP Address , One is WAN Oral IP , One is LAN Oral IP( subnet IP)!
• Router LAN Port connected host , All belong to the subnet of the current router !
• Different routers , subnet IP It's all the same ( Usually 192.168.1.1) . Hosts in the subnet IP Can't repeat , But between subnets IP It can be repeated !
• Every home router , In fact, it is also a node in the router subnet of the operator . Such an operator router may have many levels , The outermost operator router WAN mouth IP It's a public network IP 了 !
• When the host in the subnet needs to communicate with the external network , The router will IP The first IP Address conversion ( Replace with WAN Oral IP), In this way, replace... Step by step , In the final packet IP The address becomes the public network IP. This technology is called NAT(Network Address Translation , Network address translation ).

route

Concept : Find a way to the destination in the complex network structure !
IP The transmission process of a packet :

• When IP Data packets , When you reach the router , The router will check the destination first IP!
• The router decides that the packet can be sent directly to the target host , Or send it to the next router !
• Repeat the above steps , Until the end is achieved IP Address !