Shulou(Shulou.com)06/01 Report--

Network programming for introduction to Network hackers (1): network should know and should know

First, at the beginning of the design, the computer network determines its simple structure and reliable transmission. In addition, it can also connect different kinds of computers.

On the network, each node is equally important and must have redundant routes

Theoretically, the network is divided into seven layers, from bottom to top, physical layer, data link layer, network layer, transport layer, session layer, presentation layer, application layer.

In practical application, it is divided into four layers, namely: link layer, network layer, transport layer, application layer.

Link layer: transmission of data frames between devices (such as network cards, switches)

The link layer is to ensure the interconnection between devices.

Network layer: the transmission of packets between hosts, such as routers

The network layer is to ensure that each packet can reach the destination host, commonly used protocols such as IP Internet Protocol, ICMP Internet Control message Protocol, IGMP Internetwork Group Management Protocol, etc.

Transport layer: data transfer between processes on different hosts

Inter-process communication between different hosts on the network through socket sockets, commonly used protocols such as TCP (transmission control) protocol, UDP (user Datagram) protocol

Application layer: communication between applications (QQ, Wechat)

Data transfer between applications through ports, protocols such as FTP File transfer Protocol, TFTP simple text transfer Protocol, Telnet remote login, HTTP Hypertext transfer Protocol

III. TCP Protocol-- A connection-oriented network communication protocol that works at the transport layer

It provides services for inter-process communication on different hosts, usually downloading files, videos, and sending emails using TCP protocol.

Features:

1. Connection-oriented, every communication has a process of establishing-using-releasing a connection.

two。 Safe and reliable but slightly inefficient

3. The destination host must reply to confirm, otherwise the packet will be retransmitted (the theoretical basis of DOS/DDOS***)

4. The order and content of the packets are consistent.

5. Perfect error checking and retransmission mechanism

6. Each packet contains a sequence number and an acknowledgement number

7. Broadcast and multicast are not supported

4. UDP Protocol-- A connectionless network communication protocol that works at the transport layer

Together with TCP, it provides services for inter-process communication on different hosts, usually watching video online (RTP streaming service), ARP broadcasting, and DHCP,NTP,DNS using UDP protocol.

Features:

1. For connectionless, no connection is established for each communication

two。 Relatively unreliable but efficient

3. Do not check the order of packets

4. There is no perfect error checking and retransmission mechanism.

5. Each packet contains a sequence number and an acknowledgement number

6. Support for broadcast and multicast

5. IP address classification

Avatar 0.0.0.1th 126.255.255.254 has 2 to the 24th power minus 2 IP addresses

The first bit is 0, and 127.x.x.x is also a Class An address but is used for local loops

Bazaar 128.0.0.1191.255.255.254 has a total of 2 to the power of 16 minus 2 IP addresses

The top two are 10.

192.0.0.1mm 223.255.255.254 A total of 2 to the power of 8 minus 2 IP addresses

The top three are 110.

DVR 224.0.0.1' 239.255.255.254 Multicast address

The top four are 1110.

E: reserved addr

Port and port number (transport layer identification)

The port can be vividly understood as the window of the computer, and computer programs communicate with each other through the port.

The TCP/IP protocol uses the port to identify the process of communication and distinguishes multiple processes in a system.

Each running network program has a process number (PID) and at least one port number (Port)

Because the process number is randomly assigned by the system and cannot be used to send and receive data, the network process is identified by the port number.

The port number is unsigned short integer number (0,65535) Port 0 cannot be used, 1x 1023 is a known port, and some basic applications such as FTP-21,HTTP-80,TFTP-69,XSHELL-22 have occupied it.

TCP and UDP maintain separate port numbers, each with 65536 ports

Byte order

Refers to the storage order of multi-byte (more than two bytes) data, divided into small-end format and large-end format

The low-end format stores low-byte data at a low address, and the big-end format stores low-byte data at a high address.

Usually, the byte order of personal computers is in small-end format, but the byte order of communication on the network is in large-end format.

Therefore, the communication between heterogeneous computers should convert their own byte order into network byte order.

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