How can i only allow people who came from a certain site to view a web page?

im designing a gallery for a forum and i want the gallery which is on a different host than the forum to only be able to be viewed by people who clicked the link on the forums page i don%26#039;t want to have to implement a login system i just want to limit where the users can come from

some more details

the gallery is launched in an iframe when a user clicks a link on the page

How can i only allow people who came from a certain site to view a web page?

You could use an HTTP referer... I%26#039;m not actually a web designer type of person, so I don%26#039;t know the specifics, but I don%26#039;t think this system is sound. Perhaps it%26#039;d be easier just to implement password protection.