Where is the proper location to install a vent in a bath tub drain line?

Is a vent necessary? What is the real purpose of the vent?

Thanks in advance

Where is the proper location to install a vent in a bath tub drain line?
You take the vent off just downstream from the trap.

Is a vent necessary? all codes mandate that every fixture that has a trap must be vented.

What is the real purpose of the vent? Let me explain about vents. When you pull a plug or flush a toilet the water going down the pipe sets up a suction. This has to be relieved by outside air. Hence a pipe through your roof. If the vent is blocked by a birds nest or a dead critter the water will pull suction through the nearest trap and you will hear a gurgle. It may be a fixture trap or the toilet may talk to you but you will hear it someplace. While it is possible for the suction to pull enough water out of a trap to have sewer gas escape but it's more likely a smell will come from decaying food, grease or hair. . And now you know about vents.
Good luck, Tom

Thanks Speedball! (Tom)

Tom is so right. If you want to do an experiment pop the top off a pop bottle and turn it completely upside down, do you want a toilet to flush like this?