What is the difference in the following two IPv6 PTR formats?
184.108.40.206.0.0.0.0.0.0.0.0.0.0.0.0 IN PTR smtp.example.com. 220.127.116.11.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.18.104.22.168.0.a.4.f.22.214.171.124.ip6.arpa. IN PTR smtp.example.com.
If I have a valid IPv6 address and dig for the PTR it does not seem to matter which way I have the record listed in the reverse zone file. It works with either one. I would expect it to work with the nibble format (the bottom example) but I would not expect it to work with the shorter version in the top example.
What is the first example called and how can I convert the nibble format to the shorter version? I have searched and searched everywhere but can't seem to figure out this shorter format or what makes it work.