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Mexican nationals who reside in Mexico can apply for a border crossing card, which allows them to enter the US without a passport, but they must remain within 25 miles of the border (55 miles in New Mexico). The card also serves asa B-1/B-2 visa when it is presented in combination with a passport, so it is easy to confuse border crossing cards with B-1/B-2 visas.
When a border crossing card holder enters using the card and a passport, there is no restriction on where they can travel, and anyone using a "regular" B-1/B-2 visa, that is, one inserted as a sticker in the passport, is also able to travel freely around the United States after being admitted.