prerequisites

tutorial

A record

if your server can only be connected to with a full ip address (no domain), you will need to give it one. in your domain's dns settings, create an A record for some path on your website. for example, play.example.com. your A record should look something like this:
play.coolpixels.net. 1 IN A 155.94.252.232

SRV record

since 1.3.1, minecraft supports SRV records. SRV records allow server owners to host multiple servers from the same ip but for each of them to have a separate domain. create a SRV record similar to this one:
_minecraft._tcp.coolpixels.net. 1 IN SRV 10 40 25578 play.coolpixels.net.

the first parameter must begin with _minecraft._tcp., as this is the srv record minecraft looks at. this would make the minecraft ip coolpixels.net, if you want to use a subdomain just add it after _minecraft._tcp.

. for this use, the priority and weight don't matter. make sure that instead of play.coolpixels.net you put the subdomain you used for the A record.