DNS (Donam Name Service) is the system that allows us humans to find computers on the Internet by using easy to remember names. Each computer on the Internet has an IP address, which is a series of 4 numbers connected by 'dots' (.). For example, 192.168.1.45 is an IP address.

However, it is easier to remember a name rather than an IP address. So DNS was invented to map a domain name to an IP address or other DNS records. For example, the name www.saneinc.net points to the IP address for this web server. This way your web browser can connect to the web server to view the web site contained on it.

The DNS system is always in operation in the background when someone is using the Internet. Most people do not even realize it is there until they purchase their own domain name to represent themselves or an organization on the Internet. But it is an important part of how the Internet works.

The DNS 'A' record is the 'address' record. It maps a domain or host name to an IP address. For example, www.saneinc.net has an A record that maps to the IP address of that computer.

MX stands for Mail eXchange. Therefore an MX record is used to route email for a domain or hostname. It tells the Internet where to send email for that domain or hostname.

Each MX record has a number associated with it which is the 'weight' of the record. The lower the number, the higher priority that record has over other MX records for the same domain name. This allows one to setup 'backup' servers in case the primary mail server for a domain is not available.

Say for example the domain example.com has the MX records:

10 mail1.example.com
20 mail2.example.com

When someone sends an email message to someone at example.com, the mail software will first try to send it to the server mail1.example.com. If that server is not available, it will send it to the server mail2.example.com.

The PTR record in DNS is the exact opposite of an A record. The A record maps a domain name to an IP address. The PTR record maps an IP address to a domain name.

While not required, it is important to make sure the PTR record is set for a computer on the Internet. Some sites will not accept email or connections from computers that do not have an appropriate PTR record setup.

However, unlike a domain name, PTR records are under the control of the owner of IP address. This is usually a large ISP. Therefore most people will have to work with their ISP to make sure the PTR record is correct for any of their computers on the public Internet.

A registrar is an organization accredited by ICANN (Internet Corporation for Assigned Names and Numbers) to register and manage domain names. In today's Internet, most registrars are businesses which offer other services besides domain registration. There are many registrars to choose from and pricing can vary based on the services you need.

In DNS parlance, a zone is basically the file or database that contains the DNS records for a domain

The DNS system is essentially a tree like structure and the root nameservers are at the base of the tree. Any search through DNS starts at the root nameservers and works its way down to the authoritative servers for the domain being searched.

Root nameservers are run by various organizations with permission from ICANN (Internet Corporation for Assigned Names and Numbers).

Authoritative nameservers contain the DNS records for one or more domains. Essentially they maintain the zone files for a domain. You can find out the authoritative nameservers for your domain by using the tools at DNSstuff.com. Use the DNS lookup tool on the top right-hand side to lookp the NS records for your domain.

As mentioned, DNS is always happening in the background. Most DNS information does not change that often. To reduce network traffic it is possible to cache, or hold onto, information that has been retrieved from an authoritative server for a period of time. The owner of the domain or authoritative server controls how long that period of time is.

Most ISPs run caching nameservers for their customers. That is their customer's computers are configured to use the ISPs caching nameservers to lookup DNS information for them. When the data is retrieved from an authoritative server, the data is based back to the client computer asking for it. If another client computer asks for the same information, the data from the cache is based back rather than going to the authoritative nameserver again.

This is a great system but its only drawback is that it can delay how long it takes for a change to a DNS record to propagate across the Internet. Exactly how long is generally controlled by the owner of the authoritative server.