But basically for simple hosting (without SSL etc) you usually only need 2 IPs for 2 NS addresses. At least for a newbie
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I think really old hosting methods also use 1 IP per virtual host. But with HTTP 1.1 thats not really needed anymore.
One good use for multiple IPs is if you host VMs. You could get a monster server and put maybe 20 VMs on it then resell the VMs as virtual dedicated servers.
If you host a mail server you could maybe also use all the IPs so if one gets blocked by RBLs it won't take out your entire server. It will give you a chance to rectify the spam problem and get it unblocked while other customers can still send mail ok.
The proper method is at least two nameservers which are on machines seperate machines from all your other stuff, and seperate from each other. If you only have one machine and are running everything from it, you only need one nameserver. Doesnt matter how many you have if you only have one machine. The idea is to have one or more all on seperate servers so if one goes down, your sites are still accessible. It's kinda silly to have two nameservers on the same machine.
Nice to have two name servers but what if the actual host server goes down? sure names will resolve but you're still in water. Unless you replicate hosting to two servers as well but it would be impossible to make enough profit that way to pay for two servers for the capacity of one.