Originally Posted by
Swanhunter
Good point about Kazakhstan, however Russia and Belarus are a common immigration zone in that there are no controls at the border. You do need the relevant visa for hotel check in, registration etc. When I travelled the Trans-Sib I was stamped into Belarus at Brest and only received a Russian exit stamp at Nauskhki.
There are indeed no immigration and custom checks at the border, but I would stop short of saying that they have a common immigration zone. Each of them has its own immigration laws and regulation, and they both require their own visas from most foreigners. So, if someone were to enter Belarus and exit Russia, he/she would require Belorussian and Russian visas in the passport. In practice, a person with just one visa--either Russian or Belorussian--can travel within both countries, but it is still illegal and if caught, he/she would face fines and deportation.