Rialto, from the Latin Rivus altus (the high brink), was probably one of the first settlements in what is now Venice. Until the Middle Ages, Venice itself was often referred to as Rialto or Rivoalto.
Venice is divided into six sestiere whose origin is as old as the city of Venice itself. As far as we know, they have always been there.
The Birthday of Venice of is traditionally on March 25th, 421 CE, but this is far more legend than historical truth.