The Ichud HaKehillos LeTohar HaMachane anti-Internet rally is coming up in a few weeks, on May 20, 2012. What do contemporary halakhic authorities say about the permissibility or prohibition of using the Internet? Obviously it seems like some forbid and some permit. What are the different reasons for and against prohibiting it?
(For the sake of simplicity I will only address HTTP browsing and not all the different uses of the IP protocol.)
The underlying Halachic reasoning for possible forbidding the use of the Internet is simple: if we know that most (or a large part) of the Internet users end up undermining their Yiddishkeit we would declare that the Internet is a danger and one is forbidden from using it. As the Gemmorah calls it " וכל היכא דקביע היזיקא לא סמכינן אניסא" (Kid. 29)
This is based on the obligation to keep away from potential [spiritual and physical] dangers - ונשמרתם מאד לנפשתיכם (Deut 4,15).
One would wonder why the Rabbis don't forbid driving cars as thousand die and many more are wounded every year in car accidents. The answer is that we hold that spiritual degradation is far worse than physical death (Bem"R 21:4):
"רַבִּי שִׁמְעוֹן אוֹמֵר מִנַּיִן שֶׁהַמַּחְטִיא אֶת הָאָדָם יוֹתֵר מִן הַהוֹרְגוֹ, שֶׁהַהוֹרֵג הוֹרֵג בָּעוֹלָם הַזֶּה, וְיֵשׁ לוֹ חֵלֶק לָעוֹלָם הַבָּא, וְהַמַּחְטִיא הוֹרְגוֹ בָּעוֹלָם הַזֶּה וּבָעוֹלָם הַבָּא."
It should be clear that (unfortunately) the only negative aspect of the Internet that's taken into consideration is pornography, overlooking many others, such as general scientific knowledge (that invalidates many common religious beliefs), the culture of pleasure and entertainment, other religions (incl atheism) and more.
Another legitimate question is regarding the above, why NY is not forbidden to live in? The answer is that the abundance and the availability of the information make it so dangerous.
It should be noted that as the Rabbis (I mean Gdoylim) themselves don't have any Internet experience, their judgment is based exclusively on what they are told by their immediate surrounding. And it is common that they are manipulated in this way.
The current (end of 2018) situation in Israel is that most Haredi households where the husbands are Avrechim (study Torah all day) don't have a broadband Internet connection (which can cost up to $50 monthly, which is a steep price for many). Of those who do have Internet (90% for women's work), most use a removable Kosher-line cellular modem that is connected only for the time of the work so the kids won't have a chance to experience the Internet.