Glossary: DNS

DNS (Domain Name System, Domain Name System ) is a distributed , enabling users to access the Internet more conveniently without having to remember IP strings that can be directly read by machines. The process of obtaining the IP address corresponding to the host name through the host name is called domain name resolution (or host name resolution). The DNS protocol runs on top of the UDP protocol and uses port number 53. In the RFC document, RFC 2181 has a specification for DNS, RFC 2136 describes the dynamic update of DNS, and RFC 2308 describes the reverse cache of DNS query.

Each IP address can have a hostname, and the hostname (domain name) consists of one or more strings separated by decimal points. With a hostname, instead of memorizing the IP address of each IP device, just remember a relatively intuitive and meaningful hostname. This is what the DNS protocol is supposed to accomplish.

Supplement: Domain Name Structure
Usually , the general structure of Internet host domain name is: host name. Third-level domain name. Second-level domain name. Top-level domain name.
The top-level domain name of the Internet is registered and managed by the Internet Network Association Domain Name Registration Query Committee responsible for network address allocation, and it also assigns a unique IP address to each host on the Internet. There are three major network information centers in the world: Inter-NIC in the United States, responsible for the United States and other regions; RIPE-NIC in the Netherlands, responsible for the European region; APNIC in Japan, responsible for the Asia-Pacific region.

There are two ways to map hostnames to IP addresses:
1) Static mapping , each device is configured with a host-to-IP address mapping, each device maintains its own mapping table independently, and is only used by this device; host file)
2) Dynamic mapping , establish a set of domain name resolution system (DNS), and only configure the mapping from host to IP address on a dedicated DNS server. For devices on the network that need to use hostnames to communicate, you first need to query the DNS server for the IP address corresponding to the host.
The process of obtaining the IP address corresponding to the host name through the host name is called domain name resolution (or host name resolution). When parsing the domain name, the static domain name resolution method is used first. If the static domain name resolution is unsuccessful, the dynamic domain name resolution method is used. You can put some commonly used domain names into the static domain name resolution table, which can greatly improve the efficiency of domain name resolution.

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Forward resolution and reverse resolution in DNS
forward resolution: lookup ip by domain name;
reverse resolution:
by Refer to http://blog.csdn.net/jackxinxu2100/article/details/8145318