Every domain name must have a primary nameserver (eg. ns1.domain-name.com) and at least one secondary nameserver (eg ns2.domain-name.com)  otherwise they won't work! .The second nameserver requirement aims to make the domain name still reachable even if one name server becomes inaccessible. 

Dynamic DNS is a system which allows the domain name data held in a name server to be updated in real time. The most common use for this is in allowing a domain name to be assigned to a computer with a varying (dynamic) IP address. This makes it possible for other sites on the Internet to make connections to the machine without needing to track the IP address themselves.