DNS A and AAAA records are the fundamental glue that translates human-readable domain names into the numerical IP addresses that computers use to find each other, but their relationship to actual network connectivity is far more nuanced than a simple lookup.
Let’s see this in action. Imagine you’re setting up a new web server for example.com. You’ve got your server ready at 192.0.2.10 (IPv4) and 2001:db8::10 (IPv6). Here’s how you’d configure the DNS records using a common command-line tool, dig, to query them:
dig A example.com +short
192.0.2.10
dig AAAA example.com +short
2001:db8::10
When a user types example.com into their browser, their computer first asks a DNS resolver (often provided by their ISP or a public service like Google’s 8.8.8.8) for the IP address. The resolver then recursively queries authoritative DNS servers for example.com to get the A record (for IPv4) and the AAAA record (for IPv6). The browser then attempts to connect to both of these addresses.
The problem DNS solves is that humans remember names, not numbers. Before DNS, users had to maintain a local hosts file with every website and its corresponding IP address. This was obviously unscalable. DNS, with its hierarchical and distributed nature, allows for a vast, globally managed system.
Internally, when you make a DNS query, it’s a UDP packet (usually, though TCP is used for zone transfers and sometimes for large responses). The query goes to a recursive resolver. If the resolver doesn’t have the answer cached, it starts at the root servers, then queries the TLD (.com) servers, and finally the authoritative nameserver for example.com. The authoritative server holds the actual A and AAAA records.
The A record maps a hostname to an IPv4 address. The AAAA record maps a hostname to an IPv6 address. You can have multiple A or AAAA records for a single hostname, which is often used for load balancing or redundancy. The client’s operating system or browser typically tries to connect to the first IP address it receives, but may try others if the first fails.
The most surprising aspect for many is that having a valid A or AAAA record doesn’t guarantee a server is reachable or responsive at that IP address. The DNS system is purely a lookup service; it has no concept of the network path or the state of the server beyond its IP address. A server might be offline, firewalled, or experiencing network issues, yet its DNS records remain perfectly valid. This is why clients often have timeouts and retry mechanisms built-in, and why administrators need to monitor their actual server health, not just their DNS configuration.
The next concept you’ll likely encounter is the CNAME record, which allows one domain name to be an alias for another.