DNS

We learned that computers use IP addresses to communicate. Yet we have been visiting websites using names such as example.com. In everyday use, we type names like wikipedia.org or google.com rather than memorizing addresses like 192.0.2.1.

Domain names

A readable internet name such as example.com is called a domain name. Numeric IP addresses work well for computers, but names are easier for people to remember.

Computers still need IP addresses to connect. DNS (Domain Name System) provides a way to look up addresses from domain names. Think of it like finding someone’s phone number by looking up their name in a contact list.

flowchart LR
    client[Client] -->|Look up example.com| dnsServer[DNS server]
    dnsServer -->|Return IP: x.x.x.x| client
    client -->|Request to x.x.x.x| exampleServer[example.com server]

    classDef clientStyle fill:transparent,stroke:#8eb7c2,stroke-width:2px
    classDef dnsStyle fill:transparent,stroke:#5b7c6b,stroke-width:2px
    classDef serverStyle fill:transparent,stroke:#d0a476,stroke-width:2px
    class client clientStyle
    class dnsServer dnsStyle
    class exampleServer serverStyle

Inspecting DNS resolution with curl

Use curl’s -v option to see information about resolving the domain name.

curl -v https://example.com

curl.exe -v https://example.com

Near the beginning of the output, look for lines similar to:

* Host example.com:443 was resolved.
* IPv4: ...

Host example.com:443 was resolved. means curl found an address for the name. The IPv4: line lists the IPv4 addresses it found. Results can vary by network and over time.

Exercises

Decide whether each statement is True or False.

1. Domain names give people a readable alternative to remembering IP addresses.
2. DNS can look up an IP address for a domain name.
3. was resolved in curl’s output means the HTTP exchange has finished.