DNS - Domain Name System

What is DNS?

1. A distributed database implemented in a hierarchy of DNS servers
2. An application-layer protocol that allows hosts to query the distributed database

How does DNS retrieve IP addresses from from hostname?

1. The user machine runs the client-side of the DNS application
2. The browser extracts the hostname from the URL and passes the hostname to the client-side of the DNS application
3. The DNS client sends a query containing the hostname to a DNS server
4. The DNS client eventually receives a reply, which includes the IP address for the hostname
5. Once the browser receives the IP address from DNS, it initiates a TCP connection to the HTTP server process located at that IP address

Other services provided by DNS

• Host aliasing
• Mail server aliasing
• Load distribution