If you need more domain name servers (e. g. ns3.yourdomain.com and ns4.yourdomain.com), if you are a web hosting provider use this syntax to add name servers to your DNS template:

yourdomain.com. IN NS ns.yourdomain.com.
yourdomain.com. IN NS ns3.yourdomain.com.
yourdomain.com. IN NS ns4.yourdomain.com.
ns.yourdomain.com. IN A {dnsip}
ns1.yourdomain.com. IN A {dnsip}
ns2.yourdomain.com. IN A {dnsip}
ns3.yourdomain.com. IN A 99.99.99.99
ns4.yourdomain.com. IN A 99.99.99.99

Be sure to change 99.99.99.99 to the ips given to you by your reseller, or server.