Unlimited bandwidth is a deceptive lie some host tell you to get your business. All servers have a set amount of resources and when those are exceeded your site slows to a crawl or stops responding. There is no such thing as unlimited bandwidth.
What they really should be saying is they don't charge you fro the amount of bandwidth you use.
So like VirtyBox said, it will not help one single bit against a DDoS attack.
Unfortunately, Unlimited Bandwidth does not stop DDOS attacks. As mentioned above you need to use DDOS protection. Above it was said that you could use Clizdfrale which is true. Cloudflare is also free.
I would like to add that to a serious website it might not be the best option. There are countless other providers that do a better job (paid of course).
To be fair, Cloudflare is not bad, but free service will never be as good as it is necessary to protect the integrity of your website.
No, this actually has no effect on your "unmetered bandwidth" plan, this just simply means the downstream bandwidth limit isn't counted to have to charge you extra money if you go over. So if you incurr a DDOS attack you are not going to rack up a huge bill for overages But the ISP provider may choose to change your plan if you have repeated attacks that seem to jeopardize their infrastructure. Now this is just merely me giving my 2 cents, has nothing to do w/ any providers terms of service or etc.
DDOS attack is basically when hackers flood your servers with too many malicious requests, so much that your server can't handle the load and it goes down. As you can see, this has nothing to do with unlimited bandwidth. Unlimited bandwidth may protect you from being overcharged in case of DDOS but unless you put proper security measures in place, your server will go down anyway and stop the downstream bandwidth eventually.
DDoS is not done through only occupying all bandwidth, it can be done through other way like overloading your server through buggy scripts, overloading mysql etc So unlimited bandwidth itself it not a solution.
Generally speaking, with reasonably available hardware, the fastest most servers are connected at is 100Mbps, 1Gbps, or 10Gbps. 100Mbps is still the most common. 1Gbps is a reasonably affordable upgrade and/or default nowadays. 10Gbps tends to be a deliberate upgrade, as it generally requires special hardware. The concept of "unlimited" bandwidth really means "unmetered" bandwidth on one of those ports. That is, nobody is counting how much you're transferring on that port, but simple technical limitations are going to limit you to 100Mbps, 1Gbps, or 10Gbps through port speed.
I said all that to say this:
10Gbps would be a really small DDOS attack. So any hosting with "unlimited" bandwidth, even optimistically connected at 10Gbps, is probably getting taken down by a routine size-based DDOS attack. Being hosted with a capacity greater than 10Gbps is generally a custom thing and is never an unlimited thing (unless you're paying a pretty penny for that quantity of unmetered transit.)
That said, like the poster above me said, lots of other types of DDOS, such as a syn flood, can take your server offline with far less bandwidth. Such attacks would be equally vulnerable with or without "unlimited" bandwidth.