HCIA - application layer

HCIA - Common application layer protocols

Basic concepts and protocols of application layer

Application layer Overview

• TCP/IP The top level of the model
• Providing network services directly to applications （ The application layer is not an application ）

Here's the picture , Use outlook The client sends mail , It uses SMTP agreement . May be through fox Client receives mail , It's called POP3. No matter what client you use , They all call the same application layer protocol .

Common application layer protocols

• DNS
• HTTP（HTTPS）
• SMTP And POP3,IMAP（IMAPS）
• Telnet（Telnets）
• FTP And TFTP（TFTPS）
  s For safety , In general use SSL/TLS Encrypt at the transport layer

DNS（Domain Name System, Domain name resolution system ）

• establish IP Mapping between address and domain name
• Resolve the domain name to IP Address （A Record ）
• take IP Address resolution to domain name （ Pointer record ）
  www.xxxx.com.
  layered ：
• Root region ,com hinder “.”
• Top-level domain name ： First level domain name
• Two level domain name ： User registration
  Windows Some orders
  ipconfig /displaydns see DNS cache
  ipconfig /flushdns     eliminate DNS cache
  nslookup                  DNS Debugging tools

HTTP and HTTPS

• Help clients access the World Wide Web （World Wide Web）
• Web browsers translate HTML Files to represent text 、 Images 、 Music, animation, video and other objects
• HTTP There is no encryption verification mechanism , Plaintext transmission , Insufficient safety
• HTTPS Use SSL、CA Certificate and other technologies for safety protection , More and more Web Mandatory use of website
  

The mail service ：SMTP And POP３／IMAP

SMTP	Simple Mail Transfer Protocol, Simple mail transfer protocol , For sending mail
POP3	Post Office Protocol v3, Post office protocol version 3, For receiving mail
IMAP	Internet Message Access Protocol, Internet Mail Access Protocol , similar POP3, Function more

Telnet：Terminal Network, Terminal network

• Remote management protocol （ Network devices 、 The server 、 Database etc. ）
• Common terminal tools ：SecureCRT、Putty、Xshell
• Plaintext transmission data , unsafe （ It is recommended to use SSH）
  

FTP and TFTP

Stage summary