3.3.2. Name Resolution: DNS and FQDN
💡 First Principle: Computers communicate using IP addresses; humans communicate using names. DNS (Domain Name System) bridges this gap by translating names to addresses. When DNS fails, everything that uses hostnames breaks—even when IP connectivity is perfectly fine.
A FQDN (Fully Qualified Domain Name) is a complete domain name that specifies an exact location in the DNS hierarchy: webserver01.corp.example.com. The FQDN uniquely identifies a host regardless of where the DNS query originates. Partial names (like just webserver01) are resolved using the client's configured search domain.
DNS Resolution Process
- Client checks local hosts file (
C:\Windows\System32\drivers\etc\hostson Windows;/etc/hostson Linux)—entries here override DNS - Client checks local DNS cache
- Client queries configured DNS server
- DNS server resolves recursively if needed
DNS Record Types (commonly tested on Server+):
| Record Type | Purpose | Example |
|---|---|---|
| A | Hostname → IPv4 address | webserver01.corp.com → 10.1.1.10 |
| AAAA | Hostname → IPv6 address | webserver01.corp.com → 2001:db8::10 |
| PTR | IP address → hostname (reverse lookup) | 10.1.1.10 → webserver01.corp.com |
| CNAME | Alias → canonical name | www.corp.com → webserver01.corp.com |
| MX | Mail exchanger | Directs email to mail server |
| NS | Nameserver | Identifies authoritative DNS servers for a domain |
The hosts file is the first thing checked before DNS. This means a wrong hosts file entry overrides even a correct DNS record—a common source of "why can't I reach this server?" confusion.
⚠️ Exam Trap: A server that can ping by IP address but not by hostname has a DNS problem (or hosts file problem), not a connectivity problem. These are different failure modes that require different troubleshooting approaches.
Reflection Question: A server can reach 192.168.1.50 by ping but cannot connect to fileserver01.corp.com. IP connectivity is confirmed working. What is the most likely cause?
