[dns-operations] auth servers in different TLDs

[Yonghua Peng]

I saw some domains who have auth name servers in different TLDs.
such as,

;; ANSWER SECTION:
gmx.net.                84558   IN      NS      ns-gmx.ui-dns.de.
gmx.net.                84558   IN      NS      ns-gmx.ui-dns.biz.
gmx.net.                84558   IN      NS      ns-gmx.ui-dns.com.
gmx.net.                84558   IN      NS      ns-gmx.ui-dns.org.

;; ADDITIONAL SECTION:
ns-gmx.ui-dns.biz.      163883  IN      A       217.160.81.199
ns-gmx.ui-dns.com.      258650  IN      A       217.160.82.199
ns-gmx.ui-dns.de.       163237  IN      A       217.160.80.199
ns-gmx.ui-dns.org.      163744  IN      A       217.160.83.199

And this,

easydns.com.            600     IN      NS      dns4.easydns.info.
easydns.com.            600     IN      NS      dns1.easydns.com.
easydns.com.            600     IN      NS      dns2.easydns.net.
easydns.com.            600     IN      NS      dns3.easydns.org.

Is this best practices for hosting a domain?

Thank you.