Why it does not work when accessing from internet?

[Andrew]

Hello, friends,

I am building a website hosted in my office. It uses Form Authentication.
When I tested in my local network, it worked fine: Asking me to login if I
tried non-public pages. After I typed my user name and password, it directed
me to the page I wanted.

However, if a person tried to use it through internet, e.g., a machine from
our client, it stays in Login.aspx page, even after correct user name and
password were entered.

Any ideas?

Thanks a lot.