CTO Tech Blog

Like many concepts in computer networking, DNS is a simple function that is difficult to grasp initially.  DNS is an acronym for Domain Name System and the role it plays is very similar to that of a telephone book.  If we look at how computers communicate over networks or over the Internet in a very basic form, it is all based on numbers.  Like telephones, every device or host has a unique number assigned to it called an Internet Protocol (IP) address.  This can be a little confusing because your computer and mine may currently have the same IP address, but chances are that neither is connected directly to the Internet.  My computer is on a network that has a firewall connecting to the Internet.  My firewall has a unique IP address on the Internet even though my computer may not.  You could look at this like peoples’ names in that there are a number of entries for Siverns in the phone book, but if you call my number there is only one Glenn in the house.  It is not typical for someone to remember someone by their telephone number or their street address.  The designers of the Internet understood this and created DNS as a system that is similar to a telephone book for people and other computers to use as a directory to locate hosts on the Internet.

When configuring the IP settings on a computer a DNS server address is defined.  This may be done manually or, for a computer receiving its IP from a Dynamic Host Configuration Protocol (DHCP) server, automatically.  A host is also typically given a name to identify itself.  DNS servers keep records of host names and the IP addresses associated with them.  If an IP address is changed, users and computers can still find the host by name using a DNS server.  DNS can also work closely with DHCP servers to keep records up to date without the need for a system administrator to manually update them.

On the Internet there are a number of “root” servers that are used to direct traffic to other servers which hold the DNS records of hosts.  When attempting to access a host such as www.basicbusiness.com, DNS will first look for the record in the DNS server that is configured on the host.  If the record is not found the DNS server will forward the request to the root server that holds records for the root level domain .com.  This server will have a record for the domain registry server which knows where the basicbusiness.com records are stored.  The request will then be forwarded to that DNS server which will reply with the IP address of the host “www” in the basicbusiness.com domain.  You could think of this process as being similar to looking for a phone number for Glenn Siverns in Vancouver, BC, Canada.  You might go to a library to find a shelf with Canadian phone books and select the Vancouver book which would contain the number.

Many problems related to accessing web sites on the Internet can be attributed to DNS problems.  When an Internet browser reports that if is unable to find a web site, yet the computer can still receive email, the problem is very likely a name resolution issue attributed to DNS.  Here are some terms you may hear when someone is helping diagnose a DNS issue:

 IPCONFIG /all– This command will display the IP configuration of a host.  When you type IPCONFIG at a command prompt, the computer will display the IP address of the host, the subnet mask, and the gateway address.  IF the command “IPCONFIG /all” is entered, the computer also reports the DNS server addresses among other things.

PING – Similar to the use of sonic pings in submarines, ping is used to see if something responds.  Ping can be used to identify or verify a host by name or by IP address.  For example, if you type “PING www.basicbusiness.com” at a command prompt the command should list the IP address assigned to www.basicbusiness.com and send a number of packets to the IP address to see if it responds.  Note that many hosts will not respond because of security configurations, but if the ping command is able to determine the IP address then DNS is likely working.  Another useful test may be to try to ping a host by IP address if it’s known.  If the host responds to a ping by IP address, but not by name then DNS is not resolving names properly.  If neither test gets a response then there may be other network issues.

I hope this takes some of the mystery out of DNS and helps point you in the right direction next time you have a problem with accessing the Internet.