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It doesn't really matter. The point of DNS is to resolve hostnames to an IP. As long as you want whatever hostname you use to point to your server, attackers simply have to attack using the hostname.

Stressing Your own servers and websites is completly ok, (though whoever your host is might not be happy)
Stressing and trying to pull a DOS attack on someone's stuff is Completly illegal and you will face the law. 
if you want to prevent hacking (not the same as a DOS attack), use good passwords and never share them. and encrypt your data & backups (HAVE BACKUPS)

I suggest you use https://www.cloudflare.com/en-ca/ 

Your network won't be able to handle legitimate requests in time because it's too busy to handle other requests. All the other person sees on the other end is the server timed out.
 
Changing the IP address does nothing if you're using a hostname rather than an IP address to access the server. If your website is accessed via www.myhomepage.com, the DNS servers will simply resolve it to whatever IP address you set up.
The intent of the attack constitutes whether or not it's legal. If you're using it to stress test your own website, then it's perfectly fine.
As mentioned, getting DDOSed is different from getting hacked. If you really don't want to worry about being DDOSed, then use some service provider that offers DDOS mitigation like CloudFlare.

The idea is to cause so much traffic that either your router(s) can't handle it, or your bandwidth isn't enough to handle it. If you e.g. only have a subscription for, say, 50Mbps down and there's more traffic than that aimed at you, then obviously all your bandwidth will be spent on that junk-traffic.
 
Depends entirely on your ISP. Some ISPs do have equipment to deal with a DDoS, so they'll just filter it out, and some will just tell you to sit and wait.
 
Many such websites are taken down eventually, but the people behind them often slip away and just whip up a new one. Also, stress-testing your own network isn't illegal, and neither is stress-testing your employer's network if you're in charge of such operations, so that's one loophole these services rely on.
 
As for being able to prevent stuff like this? There are really only two options: 1) sign up for a business-account with your ISP and buy a shitton of bandwidth and then invest in high-end networking-equipment capable of handling a small DDoS-attack 2) sign up for Cloudflare or similar services who offer DDoS-protection.
 
Getting hacked and getting DDoSed are two entirely different things.

you know i said that to myself about 2 months ago and im still wating...