Lesson 46: Key Application Protocols: DNS (Domain Name System)

DNS is often called the 'phonebook of the internet.' It is an Application Layer (L7) protocol essential for converting human-readable domain names into numerical IP addresses.

Why DNS?

Computers and routers communicate using IP addresses, but humans prefer to use names (like coursera.org). DNS provides the translation service.

The DNS Process (The Lookup):

1. Host Request: Your computer tries to visit google.com. It first checks its local DNS cache.
2. Local DNS Server: If not found locally, the request goes to your configured DNS server (often your router or ISP server).
3. Recursive Search: If the local DNS server doesn't know the answer, it initiates a recursive search by contacting the global DNS hierarchy:
   • Root Servers: Tells the local server which Top-Level Domain (TLD) server to contact (.com, .org).
   • TLD Servers: Tells the local server which authoritative server handles google.com.
   • Authoritative Server: This server knows the specific IP address for google.com.
4. Response: The IP address is sent back through the chain to your computer, which then initiates the connection using the IP address.

Protocol Details: DNS generally uses UDP Port 53 for quick lookups, but switches to TCP Port 53 for large zone transfers.