DNS
Definition.
DNS: Domain Name System, the hierarchical and decentralized naming system used to identify computers reachable through the Internet or other Internet Protocol networks.
In More Detail
DNS maps names to addresses, and the part that catches people out is caching. Records carry a TTL, and resolvers all over the internet hold the old answer until it expires — which is why a domain change can look live for you and broken for a client on a different network.
The practical habit is to lower the TTL a day or two before a planned migration, so the switch propagates in minutes rather than hours. On launch day I schedule DNS changes with buffer time, because certificate issuance usually depends on the record resolving first and that dependency is easy to forget.
In Practice
Getting DNS records right (A/CNAME records, propagation time, and SSL provisioning) is usually the very last — and most anxiety-inducing — step of launching a client's custom domain, so I always build in buffer time for it before a go-live date.
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