There are several reasons to get a temp ban in d2, the rules are rater crazy. But it mostly boils down to making to many games to fast, the good old days just farming Meffe over and over are gone or going in and out of alot of games to mule. Temp bans normaly start with 5min and then go upwards untill hitting the maximum of 12 hours.
The way D2 works is of your IP adress. So if u got an options with your ISP to get a new Ip simply by rebooting your Router thats an option to get around it, thoe abusing this will make the rules even harder and make it even more easy get more bans.
Best way to avoid thies are simply staying longer in each game you create, do several things when farming like a few act end bosses + dia/baal runs before making a new game.