I would like to know if DOTA can be played also in LAN, without an internet connection, or if instead it relies on a permanent network infrastructure?
If you are talking about the DOTA Warcraft 3 map, then yes, it can be played on a LAN network.
More Info: DOTA is a custom map for Warcraft 3, so it can be played online or offline via a LAN network, or just yourself if you have the AI map. Like tzenes said it is faster on LAN than on the internet. GLHF
Time for value added information.
Not only does DotA work on LAN, it works better on LAN than over the internet.
Normal Warcraft 3, over battlenet, has a built in delay of 250ms. This is natural rate limiting on the responses from the server that are built in to the game. There are a number of tools you can use to get around this restriction (ie. Garena), but they step heavily on the EULA (though Blizzard has yet to come after anyone for their use).
However, Warcraft 3 on LAN reduces this delay to 100ms, making the response significantly faster.
For a game like DotA where "last hitting" and denying are crucial, playing over a LAN provides a superior experience.
As others mentioned
its possible to play DOTA on LAN.
When I was in college we used to play it by setting up an
adhoc wireless network to play, and even on that it used to work fine (no lag till 6 players: served our purpose), so definitely playing on LAN (ethernet) will be much faster. When we needed to play 5 v 5, we used ethernet in our lab ;).