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Concept ：IP Address （Internet Protocol Address） Internet protocol address .
effect ：IP The address is IP A uniform address format provided by the protocol , It assigns a logical address to every network and host on the Internet , In order to shield the difference of physical address .
form ： network number + Host number
network number ： Identify network segment , Ensure that the two network segments connected with each other have different identifications
Host number ： Identify the host , Within the same network segment , Hosts have the same network number , But there must be a different host number ;
Ensure the ip The address is unique ！

Subnet mask

In order to solve IP Address waste , The subnet mask is introduced to divide the subnet .

Format

Subnet mask format and IP The address is the same , Also a 32 The binary number of bits . On the left is the network bit , Use binary numbers “1” Express ,1 The number of is equal to the length of network bits ; On the right is the host bit , Use binary numbers “0” Express ,0 The number of is equal to the length of the host bit .

effect

Divide subnets ： You can put A~C class IP Address , Again, it is divided into smaller networks
The divisions ：16 Bit network number +1 Location net number +15 Bit host number （C class ）

The subnet mask can be based on IP Address , Calculate the network number and host number
Calculate the network number ： Subnet mask and IP Address and
Calculate host number ： The subnet mask is reversed, and then it is combined with by bit

Judge whether it is the same network segment ：
Subnet mask + The source host ip= Source network segment
WordNet mask + Purpose ip= Destination segment
The same is the same network segment , Different is not

know MAC Address

MAC Address , namely Media Access Control Address, The hardware physical address used to identify the network device .

ip Address and MAC Address

IP The address describes the overall starting and ending point of the road , Is a logical network address for people to use .
MAC The address describes the beginning and end of each interval on the road , That is, the beginning and end of each jump ; Is the network physical address used for network hardware devices .

Hop by hop network data transmission

The above figure is from the host B To host C The process of

Network nodes ： Host or router
（ The host and router have ip Address , Hubs and switches do not ）
host ： Equipped with IP Address , But devices that do not perform routing control
Router ： Both equipped with IP Address , And routing control

Network hardware equipment and technology

A hub ：

A hub is a network device that works at the physical layer , Any data sent to the hub , Simply copy and forward the data to all other ports .

Switch ：

One was maintained internally mac Address translation table , Used to record mac Mapping relationship between address and port . Data sent , Use the above mapping relationship , Through purpose mac Find the port , Send data to the host connected to the port . If you can't find it , Just send a broadcast datagram , Returned by the corresponding host mac Address .

host

Send datagram , The sending end hosts need to be encapsulated from top to bottom according to the network hierarchy ：

Source IP And purpose IP It marks the beginning and end of the whole road
Source MAC And purpose MAC Identifies the start and end of each jump
here , According to the sending end host （ The source host ） With the host at the receiving end （ Destination host ） Whether in the same network segment , To set the next hop device ：
When the source host and the destination host are in the same network segment , The next hop device is the destination host
When the source host and the destination host are not in the same network segment , The source host cannot know where the destination host is , At this time, the next hop device will be set as the gateway device （ It's a router ）

host & Router ：ARP Cache table +ARP Addressing

First ,ARP It is a protocol between data link layer and network layer ;ARP The Agreement establishes IP Address and MAC Address mapping .

ARP agreement ： Used by IP address MAC Address

ARP Cache table ： Host or router , A set of data maintained internally （ similar hash surface , adopt IP look for MAC）

At the data link layer , Find the next hop device MAC Address process , be called APR Addressing
Both the host and the router have a ARP Cache table ： adopt IP The corresponding address can be found MAC Address .
According to the of the next hop device IP Address , stay ARP The corresponding... Can be found in the cache table MAC Address , Then you can set the purpose MAC And send datagrams .
If you can't find it , Then send ARP Broadcast datagram ： Purpose MAC For broadcast address , Ask about the next hop device MAC Address .

Router ： route +NAPT

Router has two main functions ：

（1） gateway
The router acts as a gateway , It can be divided into public network and local area network , Some routers can also divide the LAN into multiple subnets （ Different segments ）
When dividing multiple subnets of LAN , You can go directly through ARP Addressing any host in the LAN .（ The LAN here is the LAN composed of multiple subnets under the router ）.
When dividing public network and local area network , When the host in the LAN sends a datagram to the host in the public network , Need to be based on NAPT agreement , Connect the LAN to the host IP Address and port number , Convert to router public network IP And port number （ Refers to the port of the program running in the router ）
There are multiple network cards in the router ： At least one public network card + Multiple subnet cards

（2） route
So called routing , That is, in the complex network structure , Find a way to the end , Routing function in router , It's like planning a route .

NAPT agreement

Intranet ip+port Convert to Public network ip+port

Conflict domain

The hosts are connected through network devices （ A hub 、 Switch ） The physical port of 、 When the network cable is connected , Two hosts send datagrams at the same time , If there is a conflict , Then the network scope is a conflict domain （Collision Domain）. The collision domain is also called collision domain in the physical layer .
After the hub receives the datagram , Is to simply copy datagrams 、 Forward to all other ports , If there are two datagrams to be forwarded at the same time , There will be conflicts . The whole hub , namely All ports of the hub are a conflict domain .
After the switch receives the datagram , Is to forward the datagram to a corresponding port ： There is no conflict when two datagrams are forwarded to different ports at the same time , But when forwarding to one port at the same time, there is a conflict . namely The switch can split the conflict domain , After split , A port is a conflict domain .

Broadcast area

Broadcast refers to that the host in a network sends data to all other hosts in the network at the same time （IP、MAC Set the address to the broadcast address ）, The range to which this data can be transmitted is the broadcast domain （Broadcast Domain）. The broadcast domain is located at the data link layer .
The hub receives a broadcast datagram , It's still a simple copy 、 Forward to all other ports , therefore All ports of the hub are a broadcast domain .
The switch receives a broadcast datagram , Will be forwarded to all other ports ; and Routers can isolate broadcast domains , That is, the broadcast domain will not spread beyond the router .