I'm trying to wrap my head around DNS. My biggest issue right now is understanding when an AAAA record is required.

Recently, I was asked by a colleague to point a subdomain from Server A to Server B.
So, I created the subdomain on Server B and edited the DNS records (via TinyDNS which is a text file based DNS system) on Server A, which has the main domain on it, to point the subdomain at Server B.

This worked fine, I and a few other colleagues could see and access the subdomain.
However, the colleague that asked me to point the subdomain could not.

It turned out that I also needed to add AAAA records to the DNS in order to "fully" point the subdomain at Server B's IPv6 address.

So what I don't get is, how could you tell when you might need to add AAAA records to the DNS? Is it a good rule of thumb to always add them?
And, most importantly, what could be the reason behind my colleague not being able to see the subdomain when I and a few others could?

  • Unfortunately, questions about protocols above OSI layer-4 are off-topic here. You could try to ask this question on Server Fault for a business network. – Ron Maupin Sep 25 '20 at 22:50

Basically, you use an A record to provide an IPv4 address for a host name, and a AAAA record to provide an IPv6 address. The client queries whichever it's interested in.

For a dual-stack host you need to add both records to make use of both stacks.

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