What is a hierarchical Domain Name System (DNS)?

Hierarchical distributed system for converting human-readable domain names (e.g., google.com) into machine-readable IP addresses (e.g., 172.217.169.142).

Key components:

Domain names: Unique names representing resources on the Internet.
IP addresses: Numeric addresses that uniquely identify devices on the network.
DNS servers: Store information about the correspondence between domain names and IP addresses.

Principles of operation:

Recursive query: The client (e.g., browser) sends a request to the DNS server to obtain the IP address for a domain name.
Hierarchical resolution: If the local DNS server does not know the IP address, it forwards the request up the hierarchy (root servers, TLD servers, authoritative servers) until it finds the required information.
Caching: DNS servers cache responses to speed up subsequent requests.

Types of resource records:

A: Conversion of a domain name to an IPv4 address.
AAAA: Conversion of a domain name to an IPv6 address.
CNAME: Mapping one domain name to another.
MX: Indicating mail servers for the domain.
NS: Defining authoritative DNS servers for a subdomain.
TXT: Storing textual information about the domain (e.g., SPF, DKIM).

Example of domain name resolution:

// User enters google.com in the browser
// Browser queries the local DNS server
// Local DNS server does not know the IP for google.com
// Request is sent to the root DNS server
// Root server responds, indicating the DNS server for the .com zone
// Request is sent to the TLD server for the .com zone
// TLD server responds, indicating the authoritative DNS server for google.com
// Request is sent to the authoritative DNS server for google.com
// Authoritative server returns the IP address of google.com
// Local DNS server caches the IP address and sends it to the browser
// Browser establishes a connection with the server using the IP address

DNS is a critically important component of Internet infrastructure, ensuring connectivity between users and resources.