What Is Domain Name to IP Address Resolution?

Domain Name to IP Address Resolution is a process to map a domain name to an IP address.

Services on the Internet are identified by two types of addresses: domain name address and IP address.

Domain Name Address uses the domain name to identify the computer that provides the service. A domain name address is easier for human to remember. For example, "http://www.cs.ox.ac.uk" is the domain name address for the Website service provided by the Department of Computer Science, University of Oxford.

IP Address uses the numeric IP address to identify the computer that provides the service. A numeric IP address is harder for human to remember. For example, "http://129.67.151.1" is the IP address of the same Website service provided by the Department of Computer Science, University of Oxford.

When you enter a domain name address into a computer, it has to perform the Domain Name to IP Address Resolution process to find the corresponding IP address in order to reach the destination computer.

Since there are so many domain names and IP addresses on the Internet, the Domain Name to IP Address Resolution is actually performed by several computers and in multiple steps.

For example, if you are trying to connect to www.cs.ox.ac.uk from your laptop, it will use 6 DNS servers to get its IP address of 129.67.151.1:

1. Local DNS (*) - The first step is for your computer to send a DNS query with the given domain name like www.cs.ox.ac.uk to the Local DNS, which is usually configured in your Internet connection setting.

2. Root Server (*.root-servers.net) - If the Local DNS Resolver fails to find "www.cs.ox.ac.uk" in its database, it will send a new DNS query to a Root Server for help. The Root Server will return a TLD Server: nsa.nic.uk, who can resolve all "*.uk" domain names.

3. TLD Server (nsa.nic.uk) - Then the Local DNS Resolver will send another new DNS query to the TLD Server given by the Root Server. The TLD Server will return a Second Level Server: ns0.ja.net, who can resolve all "*.ac.uk" domain names.

4. Second Level Server (ns0.ja.net) - Then the Local DNS Resolver will send another new DNS query to the Second Level Server given by the TLD Server. The Second Level Server will return a Third Level Server: dns0.ox.ac.uk, who can resolve all "*.ox.ac.uk" domain names.

5. Third Level Server (dns0.ox.ac.uk) - Then the Local DNS Resolver will send another new DNS query to the Third Level Server given by the Second Level Server. The Third Level Server will return the Authoritative Server: raptor.dns.ox.ac.uk, who holds the authoritative IP mapping of "www.ox.ac.uk"

6. Authoritative Name Server (raptor.dns.ox.ac.uk) - Finally the Local DNS Resolver will send another new DNS query to the Authoritative Server given by the Third Level Server. The Authoritative Server will return IP address, 129.67.151.1, which is mapped to www.cs.ox.ac.uk.

You can use "host" command on Linux systems to query DNS information.

For example, the following command returns 7 Third Level DNS servers who can resolve all "*.ox.ac.uk" domain names.

fyicenter$ host -v -t ns ox.ac.uk
Trying "ox.ac.uk"

;; QUESTION SECTION:
;ox.ac.uk.      IN  NS

;; ANSWER SECTION:
ox.ac.uk.    21599  IN  NS  dns1.ox.ac.uk.
ox.ac.uk.    21599  IN  NS  auth4.dns.ox.ac.uk.
ox.ac.uk.    21599  IN  NS  auth5.dns.ox.ac.uk.
ox.ac.uk.    21599  IN  NS  dns2.ox.ac.uk.
ox.ac.uk.    21599  IN  NS  dns0.ox.ac.uk.
ox.ac.uk.    21599  IN  NS  auth6.dns.ox.ac.uk.
ox.ac.uk.    21599  IN  NS  ns2.ja.net.

The following command returns the Authoritative DNS server who holds the IP address for www.cs.ox.ac.uk.

fyicenter$ host -v -t ns www.cs.ox.ac.uk
Trying "www.cs.ox.ac.uk"

;; QUESTION SECTION:
;www.cs.ox.ac.uk.    IN  NS

;; AUTHORITY SECTION:
ox.ac.uk.    300  IN  SOA  raptor.dns.ox.ac.uk. hostmaster.ox.ac.uk. ...

You can also use "nslookup" command on Linux systems to query DNS information.

For example, the following command returns 7 Third Level DNS servers who can resolve all "*.ox.ac.uk" domain names.

fyicenter$ nslookup -type=ns ox.ac.uk

Non-authoritative answer:
ox.ac.uk  nameserver = ns2.ja.net.
ox.ac.uk  nameserver = auth5.dns.ox.ac.uk.
ox.ac.uk  nameserver = dns1.ox.ac.uk.
ox.ac.uk  nameserver = dns0.ox.ac.uk.
ox.ac.uk  nameserver = auth4.dns.ox.ac.uk.
ox.ac.uk  nameserver = dns2.ox.ac.uk.
ox.ac.uk  nameserver = auth6.dns.ox.ac.uk.

The following command returns the Authoritative DNS server who holds the IP address for www.cs.ox.ac.uk.

fyicenter$ nslookup -type=ns www.cs.ox.ac.uk

Authoritative answers can be found from:
ox.ac.uk
  origin = raptor.dns.ox.ac.uk
  mail addr = hostmaster.ox.ac.uk
  serial = 2021080436
  refresh = 3600
  retry = 1800
  expire = 1209600
  minimum = 900

⇒ DNS Records and Record Types

⇐ DNS (Domain Name Server)

⇑ Domain Name Server/System

⇑⇑ Internet Connection - Frequently Asked Questions