CEH clearly distinguishes between active and passive reconnaissance. Passive methods involve gathering publicly available data without directly interacting with the target’s infrastructure, thus avoiding detection. Tools such as Netcraft, DNSdumpster, VirusTotal, Certificate Transparency logs, and search engine indexing are recommended by CEH for discovering subdomains through public metadata, cached DNS records, WHOIS data, SSL certificate entries, and third-party enumeration databases. These platforms provide insights into externally accessible assets without sending packets or queries to the target organization. Brute-force enumeration is active and violates the rules of engagement. Attempting credential guessing or requesting internal DNS data are unauthorized and clearly active reconnaissance activities. Passive OSINT-based subdomain enumeration is a core CEH technique used to uncover hidden infrastructure safely and legally. It is especially crucial in red team operations where stealth is a priority.