The reason is, most ISP's wont offer a sustained bandwidth at less than 1-MBit
And, that is about $1000 a month.
That's why going with an ISP that promises a limit, is over-booking, and hoping everyone won't use their limit, nd since most people try to use it at prime time, things start to back up.
When an ISP promises 1-MBit sustained, they _can't_ oversell the pipe, and they need to have EXCESS bandwidth, since you are guaranteed to have your 1-MBit anytime you need it.
They often then top off the connection with something like 5 or 10Mbit burstable. that means at any instant, you can try to grab resources above your 1Mbit, and if they are there, you'll have access to them, without being charged extra, unless your "sustained" or "average" goes above the 1Mbit for some specified time. Each ISP is different on that (when they move you to the next level of charges).
But, if you are promising 1MBit sustained, 10MBit burstable, then you can't put more than 10 servers with a 10MBit Ethernet, on a 100MBit switch. Now, since there is some sort of load balancing, and averaging, and not all sites are going to burst at the same time, the pipes get relatively smaller (even though they are larger) as you go down the line.
Does this make sense?
If an ISP is promising 1MBit/10MBit service, then on a 100Mbit line (pipe) they can put at most about 50-60 sites. That is the only way to have the "excess" bandwidth, and lack of collisions, that is necessary. As they go above that, you find less and less burstable bandwidth available.
That is _expensive_ since an ISP _can't_ over book that. They have to provide EXCESS bandwidth, not less.
Compare that to an ISP that promises UP TO 1GIG transfer per month. They _can_ over book, since there is no GURANTEE of the transfer at any time, or even thatyou'll get any of it -- just that you'll have UP TO that much before they'll charge more.
They _can_ put 200 to 500 websites on a server with a 100MBPS connection. Only one swtich, and one 100 MBPS connection. _MUCH_ cheaper, and they are charging several times for the same bandwidth -- everyone wants it at 8pm=12pm eastern, for example.
If you need a guaranteed bandwidth, the _ONLY_ way to get it is by sustained measure, not by an amount per month.
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