authentication problems

Alright all you pg_hba.conf experts . . .

I can't for the life of me figure out why I'm getting connection
error messages which mention the pg_hba.conf file. I was trying to
use the default settings:

local all trust
host all 127.0.0.1 255.255.255.255 trust

As I understand it, the first line should let any user on the same
machine as the database connect through the regular UNIX sockets
while the second line should let any user on the same machine connect
through tcp/ip (jdbc, for example). These default settings are
working great on another machine I have that's also hosting a
database. A request can come in from any ip address on the
database's machine, and the connection goes through without a hitch.

But on my problematic machine postmaster won't let me connect
(through tcp/ip using jdbc) unless I explicitly type an additional
line into the pg_hba.conf file with the machine's actual ip address
and subnet mask. Why??? It should let me in regardless of the
machine's ip.

Also, I don't understand what's special about the above numbers
(127.0.0.1 and it's mask) such that they are used to allow any ips to
come in from the same machine.

I've tried other variations beside what the docs suggest, like the
following line that was recommended:

host all 127.0.0.1 255.0.0.0 trust

but this didn't help

Any suggestions are welcome . . . or will I forever be cursed to
modify the pg_hba.conf file every time I change my machine's ip
address.

Russ