Network - network layer

One 、 Network layer functions

Address management and routing .

Two 、 Knowledge involved in the network layer

host ： Equipped with IP Address , But devices that do not perform routing control ;

Router ： Both equipped with IP Address , And routing control ;

route ： It belongs to the technology used in routers , It mainly plans the route of network data transmission （ Which direction is closer ）

node ： Host and router ;

Segment division ：

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

         network number ： Ensure that the two network segments connected with each other have different identifications

         Host number ： Within the same network segment , Hosts have the same network number , But there are different host numbers

IP Format of address ：4 Parts of , Every part is 0~255（ Decimal system ）

         classification ：A~E, There are five types of addresses, each of which has a range

Category name	network number	Host number
A	The first 1 Parts of	after 3 Parts of
B	front 2 Parts of	after 2 Parts of
C	front 3 Parts of	Last 1 Parts of

The host number range indicates the network , How many hosts can be put ;

DHCP technology ：

The new host nodes in the subnet can be automatically assigned IP Address , Avoid manual management IP, Generally, the router comes with DHCP, It can be seen as DHCP The server ;

Subnet mask ：

Format and IP The address is the same （32 Binary number ）

effect ： distinguish IP How to distinguish the network number and host number of the address ？IP Address and subnet mask are calculated by bit and , Get the network number , Negative network number , It's the host number ;

Application scenarios ： Distinguish whether it is the same network segment IP, A host sends a datagram to the destination host , Use the subnet mask to separate the hosts IP And purpose IP Get the network number , contrast , The same is the same network segment ;

special IP： The host addresses are all 1, Show broadcast （ Purpose IP It is no longer locating a host , Instead, all hosts in the network segment , Will send this datagram ） This machine ip:127.0.0.1

IP The number of addresses is limited ：

CIDR To some extent, it alleviates IP The address is insufficient （ Improved utilization , Reduce waste , however IP The absolute upper limit of address has not increased ）

Solution ：

① Dynamically assign addresses ： Only devices connected to the network are assigned IP Address , So the same MAC Address devices , Every time you access the Internet , Got IP The address is not necessarily the same ;

②NAT technology ： Be able to put private IP When communicating with the outside world, it turns to the overall situation IP, private IP And the big picture IP Mutual transformation technology

（NAPT technology ： Provide LAN host IP+ process port and Router public network IP+port Means of mutual transformation ）

③IPv6： Not at all IPv4 Simple upgrade of , These are two unrelated agreements , Not compatible with each other ,IPv6 use 16 byte 128 To represent a IP Address , But at the moment IPv6 It's not universal yet ;