Domain Name System (DNS)

One of the essential services in a network, and the backbone of the modern internet, is the Domain Name System (DNS). It provides name resolution, which translates human-readable names into IP addresses.

Slides

Names in DNS are organized in a hierarchy. A fully qualified domain name (FQDN) specifies the exact location of an entry in the DNS namespace. To understand this hierarchy you have to read a FQDN from right to left. Take the following example:

\[\text{www.hpc.temple.edu}\]

Every FQDN starts at the root. Each level in the domain has a name and is separated by a dot .

\[\underbrace{\text{www}}_{\text{Level 4}}.\underbrace{\text{hpc}}_{\text{Level 3}}.\underbrace{\text{temple}}_{\text{Level 2}}.\underbrace{\text{edu}}_{\text{Level 1}}.\underbrace{\;\;\;\;}_{\text{root}}\]

Since the root is empty, every FQDN ends with a dot .

\[\text{www.hpc.temple.edu.}\]

Note

Web browsers have omitted the trailing dot for simplicity. Try it yourself

The first level of domains are called Top-Level Domains (TLDs). This includes domains such as .com, .org, .net, etc.

\[\text{www.hpc.temple}\underbrace{\text{.edu.}}_{\text{TLD}}\]

Each domain can have subdomains. In our example temple is a subdomain of the .edu Top-Level Domain. This forms the temple.edu domain.

\[\text{www.hpc}\underbrace{\text{.temple.edu.}}_{\text{subdomain}}\]

Inside of this temple.edu domain we have another hpc subdomain.

\[\underbrace{\text{www}}_{\text{host}}\underbrace{\text{.hpc.temple.edu.}}_{\text{subdomain}}\]

Finally, the www is a hostname of a system inside of the hpc.temple.edu domain.

Each FQDN can have multiple DNS records associated to it, which can be queried from a DNS server.

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