NSDA Assistant Maintenance Engineer Exam 4 March 2022 — Question Solution | জাতীয় দক্ষতা উন্নয়ন কর্তৃপক্ষ (এনএসডিএ) সহকারী রক্ষণাবেক্ষণ প্রকৌশলী নিয়োগ পরীক্ষা ২০২২ — প্রশ্ন সমাধান

DNS (Domain Name System) is the Internet's distributed, hierarchical naming service that translates human-readable domain names (e.g. www.example.com) into machine-usable IP addresses (e.g. 93.184.216.34), and vice versa. It works like a phonebook for the Internet, organized as a tree of name servers (root -> TLD -> authoritative) rather than one central database.

Forward DNS Lookup — the common case: given a domain name, find its IP address.

• Example: resolving www.example.com -> 93.184.216.34
• Uses A records (IPv4) or AAAA records (IPv6).

Reverse DNS Lookup — the opposite: given an IP address, find the associated domain name.

• Uses a special reverse zone under in-addr.arpa (for IPv4) or ip6.arpa (for IPv6), where the IP's octets are reversed and treated as a domain name.
• Uses PTR (Pointer) records.
• Example: looking up 34.216.184.93.in-addr.arpa -> returns www.example.com.
• Commonly used by mail servers (to verify a sending server's IP matches a legitimate hostname, an anti-spam check) and in network diagnostics/logging.

sequenceDiagram
    participant Client
    participant Resolver as DNS Resolver
    participant Server as Authoritative Server
    Note over Client,Server: Forward Lookup
    Client->>Resolver: Resolve "www.example.com" (A record?)
    Resolver->>Server: Query name servers
    Server-->>Resolver: 93.184.216.34
    Resolver-->>Client: IP = 93.184.216.34

    Note over Client,Server: Reverse Lookup
    Client->>Resolver: Resolve 93.184.216.34 (PTR record?)
    Resolver->>Server: Query 34.216.184.93.in-addr.arpa
    Server-->>Resolver: www.example.com
    Resolver-->>Client: Hostname = www.example.com

Exam takeaway: forward lookup = name -> IP (A/AAAA records); reverse lookup = IP -> name (PTR record, via the special in-addr.arpa/ip6.arpa reverse zone).