yes, even my ISP had this problem 2-3 yrs back, i used to get many messages during the day of Kaspersky's Firewall blocking certain attacks, whose IP addresses were in the same range as i had, it seem to me that it's not the ISP itself, but it'd be related to some lame networking rules setup by you ISP, which allowed all the connections in your subnet to connect with each other, just like it's in normal LAN network, but after that he has extended his shop (i now see a dedicated room in his office full of CISCO swtiches, earlier it used to be just a small cabinet..
), the problem seems to have vanished, guess he has implemented a proper setup now..
.