Mail/domain name/DNS related knowledge

What is an IP address?
An IP address is a 32-bit digital ID assigned to each computer or network device on a network. On the Internet, the IP address of each computer or network device is unique across the world. The format of the IP address is xxx.xxx.xxx.xxx, where xxx is an arbitrary integer between 0 and 255. For example, the IP address of the webmaster site host is 210.22.12.54.

What is a fixed IP address?
A fixed IP address is an IP address that is allocated for a long time to a computer or network device. Generally speaking, a computer with a dedicated Internet connection has a fixed Internet IP address.

What is a dynamic IP address?
Through the modem, ISDN, ADSL, cable broadband, cell broadband and other means of Internet access to the computer, each time the Internet is assigned to the IP address is not the same, this is the dynamic IP address. Because IP address resources are very valuable, most users are through the dynamic IP address of the Internet.

   What is intranet, what is public network, what is NAT?
     Public network, intranet is two kinds of Internet access mode.
   Intranet access mode: The IP address of the computer on the Internet is the reserved address on the inetnet, and the reserved address has the following 3 kinds of forms:
   10.x.x.x
   172.16. Computers in the x.x to 172.31.x.x
   192.168.x.x
   Intranet Use the NAT (Network address Translation) protocol to access the Internet through a common gateway. Intranet computers can send connection requests to other computers on the Internet, but other computers on the Internet cannot send connection requests to computers in the intranet.
   Public network access mode: The Internet computer to get the IP address is inetnet address on the reservation. Computers on the public network and other computers on the Internet are free to access each other.
   NAT (Network address Translator) is the network addresses translation, which realizes the mutual conversion between the IP address and the public network address, and converts a large number of intranet IP addresses to one or a few public IP addresses, Reduce the occupancy of the public network IP address. The most typical application of NAT is that in a local area network, only one computer is connected to the Internet, and a NAT can be used to share Internet connections so that other computers in the local area network can also surf the Internet. With the NAT protocol, computers in a local area network can access computers on the Internet, but computers on the Internet are inaccessible to computer on the local area network.
   Windows operating system Internet Connection Sharing, Sygate, WinRoute, Unix/linux natd, and so on, are all using NAT protocols to share Internet connections. Almost all Internet access to the intranet provided by
   all ISPs (Internet service providers) is based on NAT protocol.

What is a domain name? What constitutes the domain name?
The domain name is used on the Internet to find a Web site, is an important sign on the Internet, equivalent to the host's house number. Each host corresponds to an IP address, each IP address consists of a series of numbers, such as 101.25.11.34. For the convenience of memory, people use domain names instead of these numbers to find a host, such as mydomain.com. Each domain name and IP address is one by one corresponding, people enter the domain name, and then by the Domain name server (DNS) resolved into IP address, so as to find the corresponding website. Each URL and email to use the domain name. In English international domain names, domain names can be English letters and Arabic numerals and the horizontal bar "-" composition, up to 67 characters (including suffixes), and the case of letters no difference, the maximum length of each level can not exceed 22 letters. In the domestic domain name, the three-level domain name length must not exceed 20 words.

What is DNS?
Domain name Management system DNS is the domain name resolution server. Its role in the Internet is to convert domain names into IP addresses that can be identified by the network. For example: The www.163.com that we enter when we surf the Internet will automatically convert to 202.108.42.72

What is a record?
A (address) record is a record of IP addresses that specifies the host name (or domain name). Users can point to the site server under this domain name to their own Web server. At the same time you can set your domain name level two domain name.

What is an NS record?
The NS (name server) record is a domain name server record that specifies which DNS server the domain name is parsed by.

What is an alias record (CNAME)?
Also known as canonical names. This record allows you to map multiple names to the same computer. A computer that is typically used to provide both the WWW and mail services. For example, there is a computer named "Host.mydomain.com" (a record). It also provides WWW and mail services to facilitate user access to services. You can set two aliases for this computer (CNAME): www and mail. The full name of these two aliases is "www.mydomain.com" and "mail.mydomain.com". In fact, they all point to "host.mydomain.com".

What is a generic domain name resolution?
Universal domain name resolution is defined as: The customer's domain name a.com, under the *.a.com all resolved to the same IP address up. For example, customers set up b.a.com will automatically resolve to the same IP address with a.com.

What is an MX record?
An MX (mail exchanger) record is a mail exchange record that points to a mail server that is used to locate a mail server based on the recipient's address suffix when the e-mail system sends a message. For example, when a user on the internet wants to send a letter to user@mydomain.com, The user's mail system finds an MX record of this domain name through DNS, and if the MX record exists, the user's computer sends the message to the mail server specified by the MX record (mydomain.com).

Ways to check if MX records exist
A very useful tool for DNS queries is nslookup, which you can use to query the various data in DNS. You can run Nslookup directly under Windows command line into an interactive mode where you can query for various types of DNS data.
DNS name resolution data can be of various types, with the SOA type data setting this zone parameter, with the type a data for the IP address that sets the name, and the MX type data that sets the mail exchange. These different types of data can be queried through the Nslookup interactive mode, and the set type command can be used to set the corresponding query type during the query.
Such as:
C:\>nslookup
Default Server: [202.106.184.166]
address:202.106.184.166

> Set TYPE=MX
> sina.com.cn
Default Server: [202.106.184.166]
address:202.106.184.166

Non-authoritative Answer: