An A record associates an owner name with one IPv4 address. It gets a client
to the IP layer; it does not contain a URL, port, protocol, certificate, or
health result.
Purpose and type code
IANA registers A as type code 1
[1]. Its
Internet-class RDATA is a 32-bit IPv4 address
[2]. Multiple A records at
one owner form an RRset and can publish multiple addresses.
Presentation format
www.example.com. 300 IN A 192.0.2.10The owner is www.example.com., the TTL is 300 seconds, and the RDATA is the
IPv4 address 192.0.2.10. The trailing dot makes the owner absolute in a zone
file. DNS control panels often infer the zone suffix instead.
Owner-name and coexistence rules
An A RRset can exist at a zone apex or another owner. It commonly coexists with
AAAA, TXT, and other data. It cannot coexist at an owner that is a CNAME
alias because a CNAME owner may have no other ordinary data
[3].
Changing an A record does not change delegation, registrar ownership, or an
application’s configured hostnames. Publishing an address at the apex also
does not automatically publish the same address at www.
Caching and changes
Recursive resolvers can retain the old A RRset until its TTL expires. Lowering the TTL helps only after caches holding the previous, longer TTL have aged out. Keep the old endpoint able to serve traffic during the overlap when practical.
Security and operational limits
An A answer proves only what the answering DNS path returned. It does not prove that the host accepts connections, serves the intended virtual host, presents a valid TLS certificate, or is safe. DNSSEC can authenticate the RRset when the validation chain is intact, but it does not test the service or encrypt DNS.
Avoid publishing private or unintended origin addresses. Treat an address change as availability-impacting even though reading the record is safe.
Verify
Query the exact owner and record type. Record the resolver or authoritative server, answer, TTL, and observation time. Then test the application separately over its real protocol.
Next step
Compare the AAAA record used for IPv6 addressing, or review caching and TTL behavior before changing an address RRset.