1. Everything is on the Internet.
2. The Internet contains nothing but trash.
3. Anything on the Web is findable.
4. Search engines search the entire Web.
5. The Internet is the Web.

1. Web page just posted and spiders haven't indexed it yet.
2. URL expired.
3. Web page inadequately indexed by search engines.
4. Bot blocker in effect.
5. Site not completely indexed.
6. Problems with alpha or numeric address.
7. Site is a dynamic site: permanent Web page does not exist, but is generated "on the fly."
8. The URL is case sensitive.

            secondary               file file
             domain   directory     name type
                |          |   sub   |    |
  transfer      |     top  |   dir-  |    |
  protocol      |    level | ectory  |    |
     |          |    domain|    |    |    |
     |          |      |   |    |    |    |
    http://www.census.gov/acsd/www/sub_p.htm 

Internet addresses are really a series of numerals, but can be aliased with easy-to-remember alphabetic addresses. These alpha domains are usually easy to distinguish. A list of all domains can be found at: Domain Name Registries Around the World. Common domains include .edu (US educational), .gov (US government), .org (US organizations, usually non-profit), .int (international organizations), and .us (US state and local entities).