DNS

It maps Domain name to IP address.

1. Root DNS Server

   It returns IP address of "com", "org", "io", etc.

2. Top-level DNS Server

   In com Top-level DNS Server, we can find apple.com.

3. Authoritative DNS Serve

   In apple.com Authoritative DNS Serve, we can find www.apple.com.

server {
    listen 80;                       #监听80端口
    server_name  time.geekbang.org;  #主机名是time.geekbang.org
    ...
}

Caching

Once we visited a website, its IP address is cached.

1. Browser

2. Operation System

   OS can not only cache the IP address of websites, but also that of local files. In hosts file, we can custom the domain name of local files.

3. DNS

   There are third-party DNS, too. It helps reduce the pressure of main DNS server. There are "8.8.8.8" (Google), "4.2.2.1" (Microsoft), etc.

   resolver 8.8.8.8 valid=30s;  #指定Google的DNS，缓存30秒
   

Load balancing based on domain name

If the server of "buy.tv" need maintain, we can switch to another server by telling DNS, mapping "buy.tv" to new server's IP address "4.5.6.7" instead of current IP address "1.2.3.4".

One domain name is mapped with multiple server machines.

What's more, we can create domain name of inner services. For example, we can create "mysql.inner.app" is for MySQL database service.