Castes Rai and Limbu are known to be called Kiranti. But once, it is possible those two castes were one tribe.
As their physical characteristics is quiet Mongolian it is clearly seen that they are from north. North Tibetans call Limbus Monpa meaning those who live in South of the Himalayas.
Sources reveal that the Kathmandu Valley was under the rule of Kirantis from approximately the first century BC to the second century a.d. and according to Gopal Vansawali about 32 Kiranti Kings ruled the Valley. It is also mentioned that one of the Kiranti Kings participated in the Mahabharat war.
Later the Southern immigrants Lichhavis were driven from their homeland in Vaishali in Bihar India. They came to seek the protection of Kiranti kings and were provided shelter and protection. Later, they and the Guptas over threw the Kiranti people who were forced out of their kingdoms from the Kathmandu Valley to Eastern Nepal.
Limbus are considered the first settlers of east Nepal (Limbuwan) and are thought to be descendants of the Kiratis.
Limbuwan is now part of Nepal, but it was independent until 1774