History of Mongolia
Although the Xiongnu finally had been split into two parts in AD 48, the Xianbei (or Hsien-pei in Wade–Giles) had moved (apparently from the east) into the region vacated by the Xiongnu. The Xianbei were the northern branch of the Donghu (or Tung Hu, the Eastern Hu), a proto-Mongol group mentioned in Chinese histories as existing as early as the 4th century BC. The language of the Donghu is believed to be proto-Mongolic to modern scholars. The Donghu were among the first peoples conquered by the Xiongnu. Once the Xiongnu state weakened, however, the Donghu rebelled. By the 1st century AD, two major subdivisions of the Donghu had developed: the proto-Mongolic Xianbei in the north and the Wuhuan in the south.
Origin of the Huns
The chief piece of evidence linking the Xiongnu to the European and Iranian Huns is the similarity of their names. Supporting evidence is provided by historical records indicating that the term "Xiongnu" was used for the people referred to in Sogdian and Sanskrit texts as the "Xwn" and "Huṇa" respectively, terms used for peoples called Huns in the West. Another important connection is the use of similar metallic cauldrons by the European Huns and the Xiongnu. There remain a number of outstanding differences, however, including generally different archaeological profiles and a wide variety of customs attested among the various Hunnic groups. Additionally, there is a two hundred year gap between the last recorded activities of the Xiongnu and the first appearance of the Huns in Europe and Central Asia. These issues have caused some scholars to reject the identification.