1. Why are some web addresses not accessable when keyed in with "www?"
Example: <www.strsd.southwick.ma.us> does not work;
<http://strsd.southwick.ma.us> does.

This newsgroup thread may be of interest to you:

http://groups-beta.google.com/group/microsoft.public.win98.gen_discussion/browse_frm/thread/45f2d1f4b6505255

Expanding on the newsgroup post from the previous comment post. Beyond
 DNS, web server configuration comes into account. True DNS is a
higher level to accomplish the same task, but most web servers can be
configured to listen to all HTTP requests for a block of IP addresses
(Subnets).

The web server can be configured explictly to interpet requests to
handle or deny requests with www. (or anything else) in front of the
domain name. A url is structured by
protocol://host.domain.extentsion/path . If the domain/server admin
decides that any host preceded with a www. refers to that host as well
it will work. Otherwise, it will not.