Perhaps a dumb question, but why is there only “one” Internet? Why aren’t there two, three, or four “other” Internets, wherein www.abc.com in one would be totally separate from www.abc.com in another? Sort of like parallel universes that can compete with each other.
There are other networks. The military has a global, classified network that remains completely separated from the internet. Most companies have a private intranet. AOL and Compuserve operated independent networks. There may be others I’m missing.
However, from a commercial perspective, it makes sense to have everyone on the same network. Same reason we don’t have competing phone systems.
You need to explain what you mean by “two networks”. I could easily argue that we do in fact have several networks right now. One of them is the phone system. Seriously! Think of the things that the phones can do: You can share a conversation among several phone numbers. Text messages. Faxes. Lots of stuff!
If you want two distinct "www.abc.com"s, then you’re going to need some way of keeping them separate. That means two totally separate physical sets of wire. I was going to say that this is too expensive, but that’s pretty much exactly how the internet grew around the phone lines.
Or you can have a prefix. Like how http is separate from https.
The most you think about it, the more you’ll see that we do in fact already have what you’re looking for.
Because communication networks are generally more useful the more things they are connected to. And since there’s mostly no charge for accessing internet content, there’s no commercial reason for one part of the network to deny its users access to other parts.
Of course, before about the mid- 90s, for many people “going online” meant logging on to a proprietary, private online service such as CompuServe or AOL. You generally couldn’t get to one service’s content from another, so they were sort of like competing, separate internets. The Internet, with its free access for all ethos, changed all that.
There aren’t more? Then why, in rural Missouri, do they keep referring to ‘The Internets’?
Well, that’s not actually true. You simply need a way to maintain unique machine addresses.
Crafter_Man, you’re conflating the “internet” with the “world wide web” (or, perhaps more accurately, DNS). There’s only one internet because – for correct operation – any machine that is part of the network has to have a unique machine address (ignoring NAT and such as needlessly complicating the general explanation).
That’s a technical answer. On preview, from a more functional perspective, I also like the answer given by BDoors.
One internet. Lots of tubes.
Or one internet, lots of newbs. 
Internet really means network-of-networks. Its been collected into a global system because that is what is useful to the vast majority of people. There is no commercial pressure to develop a 2nd internet seperate from the existing one. No-one would pay to set up their content on it or connect to it, when there are vast numbers of users on the existing one.
There are also plenty of private computer networks out there as well (mostly owned by businesses and governments), and not all of them have a connection to the public internet.
There’s also this:
Internet2 is just one of thousands of networks connected to the global Internet. It really isn’t any more “separate” than a large commercial network with its own circuits, like UUnet. One difference is that Internet2 does run a lot of experimental networking technologies in addition to routing ordinary IP traffic.
…which has nothing to do with networking.
I thought it only routes traffic between member institutions.
:dubious: We have lots of competing phone systems, since 1984. Right now I have the choice of using AT&T, Cox, or Verizon for my landline (and probably others that I don’t even know about), any company I want for long distance, and at least three different companies for mobile. There is one source for assigning phone numbers, but having a system to split up the numbers is not the same as having a single phone system.
If you can call Verizon from Cox they are not very separate.
Those are competing phone companies. We only have a single phone system. Even the cell network connects to the telephone network via the cell towers.
The analogy for the internet are ISPs. Each ISP is like a separate phone company. And each ISP connects to the same internet, even if the IP space (analogous to your phone number) is divided among them.
Beyond that:
QFT
But if you have a phone, you can call any other phone in the country. For there to be separate phone systems, you’d be unable to call someone who was on another phone network. It’s possible to have isolated phone networks that are essentially glorified intercoms. But if your phone can make and recieve calls to outside phones, then it’s part of the phone network.
Same way with another internet. If systems on one internet can connect to systems on another internet, then they aren’t separate internets anymore, they are one internet. The internet is just a collection of networks. If your network can connect to the outside networks, then it’s part of the internet. If your network is isolated, then it’s an intranet. It’s possible to have a really large isolated network that isn’t allowed to connect to the internet. If that network got really really really large, it might be fair to call that another internet. But as soon as you let computers on one internet communicate with computers on the other internet, then you don’t have two internets anymore, you have one internet.
As to why there’s only one system of IP numbers and domain names, well, that’s just the system we use to allow computers to network with each other. We could certainly use other systems, but this is the system we inherited. We could set up an alternate parallel system to allow computers to communicate with each other, but that would cause a lot of extra work, and that alternate internet protocol wouldn’t be a separate internet, it would be just another way of accessing the internet.
It does, and that’s no different than any other Internet-connected network. My ISP only routes traffic between me and their other customers. Anything else goes to their ISP (being a rather huge ISP themselves, they actually have multiple providers, one of which is a Tier 1, but you get the idea.)
Internet2 is just another ISP connected to each institution’s edge routers.
But your home computer can’t talk to an I2 computer. It doesn’t connect to the “internet”, there is no higher ISP.
There is no such thing as “an I2 computer.” I2 is a network. And I can and do connect to computers on that network all the time. I can’t do it via I2, since I don’t have a connection to that network and no commercial ISPs peer with them.
When a guy at Big National Laboratory needs to share a 5TB dataset with another dude at Big State University, he doesn’t press a magic “I2” button on his computer, or walk a disk over to a special “I2 computer” to send it. He simply sends the file and his institution’s routers route it over the correct network. The use of I2 is completely transparent, because it is just an IP network. That’s what it’s for.
ETA: If it helps, think of I2 as a really, really big WAN with dedicated fiber (instead of a VPN, for instance.) Your corporate WAN is still “part of the Internet,” even if it’s behind a NAT gateway and your router has one connection to a VPN to talk with other offices and another connection to the public Internet.