It so happens that the same site is available to subscribers of one provider, but users of other. The reason is that a chain of intermediate nodes through which packets from the server to the machine user, is different. To determine which nodes are involved in Linux you can use a traceroute utility, and in Windows with tracert. And if one of them is unavailable, "reach out" to the server is impossible. Then you have to use the services Skweezer, Google GWT, Opera Turbo or similar, but in any case not anonymizing proxy servers - in this case, the site owner can take you for an intruder!
The page may not open because of a typo in its URL. Then I get a message saying that the file was not found - the so-called "error 404". In the case if typo is allowed in the domain name, you may hit on a fraudulent resource. Often it looks like real, and then the user enters his data from his account. It threatens abduction of such data by cybercriminals.
Even if you correctly typed in the full url of the page it will not appear if the site owner had accidentally deleted it. Also it can move, and then all the old links to this material will be invalid. Moreover, often in haste webmaster forgets to fix such links even on other pages of their own resources.
When the hosting provider performs the prevention, not be available all or some of the sites that it services. Individual resources also become unavailable when they are updating the owners. But if a link to a small online post on another, large, it begins to move a significant number of users. The server is overloaded, and a so-called Slashdot-effect. Its name he received in honor of major site Slashdot, posting links which often leads to this effect.
Subscribers some ISPs are faced with situations when access the web page the major resource is blocked under the pretext of the availability of extremist material, though in fact nothing of the sort on these pages no. Not to allow any content provider is required by law, only to fulfill this obligation, he may negligently and instead of separate pages to block whole domains of the second level.