DNS and domains
A and AAAA records
The DNS records that give a name its IPv4 (A) and IPv6 (AAAA) addresses. Every website and every server starts with these.
An A record maps a name to an IPv4 address; an AAAA record maps it to an IPv6 address. A name can have several of each, in which case clients pick one, which is a cheap way to spread load. A name that should work for everyone has both kinds.
They are the records behind most of what you type into a browser. example.lu and www.example.lu usually point at the web server; mail.example.lu at the mail server, named by the MX record; ns1.example.lu at a nameserver. The bare domain must use A and AAAA records (or an ALIAS), because it cannot be a CNAME.
When you move a service to a new server, the A and AAAA records are what you change, and the old server should keep answering until their TTL has passed everywhere.