A Record Lookup

An A record (Address record) is the DNS entry that maps a hostname to an IPv4 address. It is the record type that makes "example.com" resolve to something like "93.184.216.34" so a browser knows which server to connect to.

A records are the oldest and simplest record type in DNS: one name, one 32-bit address, nothing else to configure. IPv6 addresses use a separate record type, AAAA, defined by RFC 3596 - an A record only ever holds a dotted-quad IPv4 address.

• A domain can publish several A records for the same name at once
• A records cannot coexist with a CNAME on the same name (RFC 1034 section 3.6.2)
• Every A record carries its own TTL, which controls how long resolvers may cache it

When a name has more than one A record, resolvers typically return all of them, and the requesting application picks one - often the first in the list, though some stub resolvers rotate or shuffle the order they receive from the server. Because most authoritative nameservers rotate the order they answer with on each query, this is called round-robin DNS.

Round-robin is a crude form of load distribution, not a health check: DNS has no idea whether a server behind an A record is actually up. If one of three A records points at a dead server, roughly a third of requests still get routed there until someone edits the record. Real failover needs monitoring outside DNS, or a provider that pulls unhealthy records automatically.

A worked example: a domain fronted by two web servers might publish

A CNAME record aliases one name to another and lets DNS do the resolving in a second step. It is convenient, but RFC 1034 forbids a CNAME from sharing a name with any other record type - including the required SOA and NS records that exist at the zone apex ("example.com" itself, as opposed to "www.example.com"). That is why you cannot CNAME a bare apex domain to something like a load balancer or a platform such as GitHub Pages: the apex must carry A or AAAA records directly.

The usual workarounds are: point the apex at the provider's IP addresses with plain A records if they publish stable ones, use a provider-specific "ALIAS" or "flattened CNAME" record that resolves the target at query time and republishes it as an A record, or run the site on "www" and 301-redirect the apex. "www.example.com", not being an apex name, can freely be a CNAME instead.

The TTL on an A record is a trade-off between DNS query load and how fast a change propagates. A long TTL (3600s or higher) gets cached further along the resolver chain and cuts lookup traffic, but a mistake or a planned migration takes that long to fully take effect everywhere, since resolvers holding a cached answer will not re-query until it expires.

A short TTL (60-300s) is standard practice to prepare for a migration: lower it a day or two ahead of the change so caches turn over quickly, make the cutover, then raise it back once the new address is confirmed stable. Some DNS providers advertise "instant" propagation for their own dashboard, but that only affects their own authoritative servers - every resolver that already cached the old TTL keeps answering with the old address until it expires, regardless of provider claims.

This tool queries the domain directly against a resolver set (Cloudflare and Google), and for an A lookup the result lists each returned address as its own row alongside the TTL reported by the authoritative server for that record. If a domain has no A record - common for domains that only serve IPv6, or subdomains kept purely for other record types - the lookup returns no rows for A rather than an error, and the domain may still resolve if it holds an AAAA record instead.

If nothing resolves at all and the query reports a failure rather than an empty result, that usually means the domain itself does not exist or its nameservers are unreachable - worth cross-checking with a nameserver lookup before assuming the A record is simply missing.