In this common case:
ns1.domain.com: IP1 ns2.domain.com: IP2
What I know the two IP addresses used like if one unreachable, the other will use as a secondary choice and user still can access the same server.
(1) I am not really understood how two IP addresses (can be more than two) is useful and making sure network up time 99% because both IPs pointed to the same server (I think both may have same physical network path or am I wrong?) Anyone can explain?
(2) Now I have I am using Rackspace Cloud Server and I maintain a cPanel. Because of limited IP v4 availability, I cannot buy an IP except for HTTPS and load balancing. So currently, I only have one IP assigned to my cPanel server.
Case 1: cPanel has built-in DNS. cPanel user with the domain pointed to this server directly are able to manage the DNS records in the cPanel UI, it will autogenerate CNAME for cpanel., webmail., ftp. subdomain access, but remember since I only have one IP address, my 1st and 2nd nameserver defined in the cPanel WHM pointed to the same IP:
ns1.mycpanelserver.com: IP1 ns2.mycpanelserver.com: IP1
Case 2: However Rackspace has DNS server virtually located just outside from the cloud. If a domain pointed to this DNS, I can manage the records from Rackspace client area and A record can be set to my cPanel server IP address to serve the hosted web site. By this way, I have two IP addresses for their respective nameservers provided which is like the common case, but I don't have the ability to edit the DNS records in cPanel UI and CNAME records must be defined manually each time new domain registered.
I am trying to use the case 2 which is common, but not sure what is the strong reasons why common two IP addresses for two nameservers in one server?