I use my own ns1/ns2 for DNS delegation with ARIN. I edit rDNS per /24, per zone file via cPanel/WHM or SSH console. The zones are created via WHM interface, easily for me.

Now, recently I acquired a legacy block of IPv4 from an ISP. Because it's legacy, they're only able to do a reallocation in ARIN to my account and DNS can only be updated on the full subnet size, let's use /20 for example. I cannot do the usual per /24 DNS delegation in ARIN just because they are legacy IPs.

So my question is, if a client asked for full DNS delegation, like the example below, how would I set this up for them?

Whether it’s a /24 or larger, what would the dns zone file look like?

Below is an example of one created by cPanel and I update for individual IPs per /24, however a full DNS delegation I’m not sure how to do once I update the NS for the /20 with ARIN to point to ns1/ns2.mydomain.com

Example DNS zone for individual IPs on a /24

In summary, I will have to do full DNS delegation on the whole /20 legacy block to ns1/ns2.mydomain.com and then on my DNS server, will have to server /24 or larger DNS delegation per client as needed. This is where I do not know what the zone file should look like.

You can assign subnets of the larger one you received. All that would be necessary is break it out in your routing and establish appropriate dns. For example, you received the subnet 111.222.32.0/20 which consists of IPs 111.222.32.0 thru 111.222.47.255. You then could assign 16 /24 subnets from that such as 111.222.47.0/24, 111.222.46.0/24, etc.

Create the zone files according to how that is broken out. You would have to be careful and use the appropriate subnet boundaries but not really difficult. There are lots of resources on the internet to help with defining subnets.