DNS and Public DNS Servers:

DNS Importance:

• Functional DNS (Domain Name System) is crucial for a functional network.
• ISPs usually provide a recursive DNS server to resolve domain names to IP addresses.
• Businesses often run their own DNS servers for internal name resolution, like naming internal hosts.

Public DNS Servers:

1. Backup and Troubleshooting: Having a backup DNS option is useful in case of DNS issues. Public DNS servers can be used for troubleshooting.
2. Building Networks: During network building, public DNS servers can serve temporarily until your own DNS server is ready.
3. Public DNS Providers: Organizations provide public DNS servers, offering free name resolution to the public.
4. Level 3 Communications: They operate public DNS servers with IP addresses 4.2.2.1 through 4.2.2.6. These addresses have been available for public use but not officially acknowledged.
5. Google Public DNS: Google provides public DNS servers with IP addresses 8.8.8.8 and 8.8.4.4. They are well-documented and acknowledged by Google.
6. Anycast: Many public DNS servers use anycast, providing global availability for better performance.

Considerations:

• Reputation: Always use DNS servers from reputable organizations to avoid potential hijacking of DNS requests.
• ISP DNS: Whenever possible, use your ISP's DNS servers for normal use, reserving public DNS for troubleshooting.
• ICMP Echo Requests (Ping): Most public DNS servers respond to ICMP echo requests, making them suitable for testing general internet connectivity using the ping command.