DNS
DNS translates domain names into technical addresses: A for IPv4, AAAA for IPv6, MX for mail servers, TXT for proofs and policies. All reachability of a domain is decided there.
In practice
Managing it deserves the rigor applied to code: documented zones, reasoned TTLs — short before a migration, long in steady state — tracked changes. DNS failures are cruel because they mimic every other failure: the site appears down, emails appear lost, when only the translation has gone astray. Always the first diagnostic reflex.