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If you go to menu>settings>wireless & networks>tethering & portable hotspots you will see the settings to enable either.
If you do that before you check with your carrier, One of four things will happen.
1.) It turns on and you tether successfully with no additional charges.
2.) It turns on and you tether, but are notified by your carrier that you now are going to be assigned the appropriate data plan if they continue.
3.) You are given a warning about needing an additional plan to tether and you cannot proceed until you activate that service with the carrier, or
4.) The tethering option has been removed by the carrier and replaced with their own app.
It is prudent to check first.
BTW, this is primarily a U.S. issue as most international carriers permit tethering without additional data plans.