Which is required for transfer of Gen III Pokemon to Gen IV (meaning you can't do on later versions of the DS).
The internet encryption security on the DS Lite requires it to be WEP to work; if, like me, you have WPS encryption, you can't connect without more effort on your part. One option is to buy a dongle and link through that (though that itself requires some effort juggling to get to work).
However, if you have a gander at your wireless router, it may well have its settings webpage url listed on the box. It's worth having a look at that, as mine had an option to set up a "guest network" via whatever encryption you wanted; so I set a new guest netwrok with a WEP encryption and, as they say, Bob's you're uncle.
The later DS versions do have the same problem, as they have slightly more encryption options, as I understand it.