Software Designer (Computer Networks)

computer network

• Seven-story model (bottom to top)

  • physical layer
    • Bit stream for transmission, which solves the interface problem between data terminals and communication equipment Data unit; bit
    • Equipment: repeaters, hubs (multi-way or multi-port repeaters)
    • Mechanical characteristics, electrical characteristics, functional characteristics, regulatory characteristics
    • Broadcast domains and collision domains cannot be isolated. Both all ports are a collision domain, (shared broadband) all ports are a broadcast domain.
  • Data link layer (there is a local area network in the middle of the above)
    • Responsible for data transmission of adjacent nodes Data unit: frame
    • Equipment: bridge (between physical layers), switch (multi-port bridge), network card
    • Basic functions: encapsulation into frames, transparent transmission, error detection
    • Broadcast domains cannot be isolated, but collision domains can be isolated. Each port is a collision domain, and all ports are a broadcast domain.
  • Network layer
    • Routing (selecting the best path) and packet switching, network congestion, heterogeneous network interconnection
    • Equipment: routers, Layer 3 switches
    • Data unit: packet (ip datagram)
    • Representative protocols IP, ICMP, RIP, ARP, RARP
      • ARP address resolution protocol, which converts IP addresses into physical addresses (mac addresses)
      • ARP light-speed cache, broadcast request, unicast response
    • Broadcast domains and collision domains can be isolated. Each port is a collision domain, and each port is a broadcast domain.
  • transport layer (transport layer)
    • Realize the communication from the sending end to the receiving end, and error detection;
    • Data unit: segment (message segment TCP or user data segment UDP) three-way handshake
    • Device: Gateway
    • The service access point is a port, representing the protocol TCP, UDP
      • UDP features
        • Connectionless, best-effort delivery, message-oriented and no congestion control, small header overhead only 8 bytes
      • TCP
        • Connection-oriented transport layer protocol, providing reliable delivery services, providing full-duplex communication, byte stream-oriented
        • Reliable transmission, connection management, flow control (variable size sliding window protocol), port addressing (UDP is also possible), congestion control
  • session layer
    • Manages and coordinates communication between various processes on different hosts, and is responsible for establishing, managing, and terminating sessions between applications.
  • presentation layer
    • To deal with the representation of the data encoding flowing through the node, the meaning of the data can be interpreted according to the service, and the application layer can read it out.
  • application layer
    • Provide services directly to the application process
    • Device: Gateway
    • protocol:
      • In UDP: DHCP(67), TFTP(69), SNMP(161), DNS(53) Below is IP
      • In TCP: HTTP(80), SMTP(25), POP3(110), TELNET(23), FTP(20/21) transmission/connection
        • Email ServiceE-mail
          • Client/Server mode (Client/Serve)
          • SMTP Simple Mail Transfer Protocol to send emails, only ASCII code text and literal attachments can be sent
            • More attachments can be transmitted using the MIME email attachment extension type PEM private email
          • POP3 is a simple plain text protocol that mainly receives emails. Each transmission is based on the entire E-mail, and partial transmission cannot be provided.
        • DHCP Dynamic Host Configuration Protocol
          • Function: Centralized management, automatic allocation of IP addresses, enabling hosts in the network environment to dynamically obtain information such as IP addresses, Gateway addresses, DNS service weapon addresses, etc., and can improve the utilization rate of addresses
          • The DHCP client can obtain the local IP address, DNS server address, DHCP server address, and default gateway address from the DHCP server.
          • Windows invalid address: 169.254.xx Linux invalid address 0.0.0.0

• URL

  • Protocol name://hostname.domain name.domain name suffix.domain name category/directory/webpage file
  • org: various organizations (non-profit teams) http default can be omitted
  • TLDs are geographic patterns
    

• browser

  • DNS domain name query sequence: local hosts file = "local DNS cache = "local DNS server = "root domain name server
  • After the main domain name receives the domain name request, the query sequence is: local cache = "local hosts cache = "local database = "forwarding domain name server

• website address

  • The structure and type of IP address (IPV4)
    • The IP address is a 32-bit binary number, four bytes, composed of a network number and a host number, ranging from 0-255
      • Digits of network number n+ digits of host number m=32
      • The number of network numbers determines the number of networks that can be allocated (2^n)
      • The number of host numbers determines the maximum number of hosts in the network (2m-2) network address broadcast address minus
    • There are five types of ABCDE for IP addresses, and ABC is commonly used
      • Type A: start with 0, decimal range: 0-127
      • Class B: 10, 128-191
      • Class C: 110, 192-223
      • Class D: 1110, 224-239
      • Class E: 1111, 240-255
      • Topic: 192.2.2.2 Only look at what category the first group corresponds to. Class C; 11010000 is also Class C
  • special IP address
    • Network address: the host number is all 0
    • Broadcast address: the host number is all 1
    • Subnet mask: the network part is all 1, and the host number part is all 0: 255, 255, 255, 0
    • Reserved address: In order to meet the needs of intranet use, reserve the ip address that is not used on the public network
      • 10.0.0.0-10.255.255.255
      • 172.16.0.0-172.31.255.255
      • 192.168.0.0-192.168.255.255
  • Subnet and subnet mask
    • The utilization rate of the two-level ip address space is low, so it is divided into three levels: network number, subnet number, and host number
      • The number of subnet numbers is 2 to the power of the subnet number
  • IPv6
    • Compared with IPv4, the address is extended, and the length of the address is 128 bits, which is the 96th power of 2 in IPv4
    • 16-bit group, 8 groups, each 16-bit is converted into 4-digit hexadecimal numbers, each group is separated by:

• Wireless network: Bluetooth has the smallest coverage and the shortest communication distance

• windows command ipconfig version

• Use the ping command to detect the network, from near to far: ping127.0.0.1=”ping local IP=”ping default gateway=”ping remote host

• Nslookup can be used to diagnose DNS failures, netstat display information, tracert route tracking packet path

• routing

  • When the server receives an IP data packet, it first searches for the host route, and then searches for the network route (direct route and remote route) before searching for the default route.
    

  • If the router receives forwarding from multiple routing protocols, it uses the routing information provided by the routing source with the smaller administrative distance.
    

• Linux

  • Only one root directory is represented by /
  • Correct a permission setting with the chmod command

• expand

  • Three networks in one: telecommunication network, radio and television network, Internet
  • Availability of the network refers to the percentage of time that users can use the network
  • In the ASP built-in object, the response object can modify the value of the cookie
  • Adding the host record of the web server in the DNS startup cycle can ensure domain name resolution and achieve load balancing
  • Two ipv6 nodes communicate through the ipv4 network using tunnel technology
  • Pure ipv6 communicates with pure ipv4 using translation technology
  • With dynamic address allocation, the local computer should be set to automatically obtain an ip address
  • The 3G communication standard independently developed by China is TD-SCDMA