Which OS of the *nix family is more common on real servers?

Question

System administration

I want to practice setting up DNS/DHCP/Mail/FTP and other network services.

Answer 1

They are set up the same way. If you can do it well on one distribution, then you can do it on any other without any problems.
And on real servers there are all popular supported distributions, which ones depend on the tasks.

Answer 2

Now the two main distributions in production are ubuntu and redhat with its free clones in the form of the same centos