<P> Various U.S. Federal government agencies use definitions that differ from their local counterparts' nine - county definition . For example, the Federal Communications Commission (FCC) which regulates broadcast, cable, and satellite transmissions, includes nearby Colusa, Lake and Mendocino counties in their "San Francisco - Oakland - San Jose" media market, but excludes eastern Solano county . On the other hand, the United States Office of Management and Budget, which designates Metropolitan Statistical Areas (MSAs) and Combined Statistical Areas (CSA) for populated regions across the country, has five MSAs which include, wholly or partially, areas within the nine - county definition, and one CSA which includes all nine counties plus neighboring San Benito, Santa Cruz and San Joaquin counties . </P> <P> Among locals, the nine - county Bay Area can be further divided into five sub-regions: the East Bay, North Bay, South Bay, Peninsula, and the city of San Francisco . Although geographically located on the tip of the San Francisco Peninsula, the city of San Francisco is not considered part of the "Peninsula" subregion, but as a separate entity . </P> <P> The "East Bay" is the densest region of the Bay Area outside of San Francisco and includes cities and towns in Alameda and Contra Costa counties, centered around Oakland . As one of the larger subregions, the East Bay includes a variety of enclaves, including the suburban Tri-Valley area and the highly urban western part of the subregion that runs alongside the bay . The "Peninsula" subregion includes the cities and towns on the San Francisco Peninsula, excluding the titular city of San Francisco . Its eastern half, which runs alongside the Bay, is highly populated while its less populated western coast traces the coastline of the Pacific Ocean and is known for its open space and hiking trails . Roughly coinciding with the borders of San Mateo county, it also includes the northwestern Santa Clara county cities of Palo Alto, Mountain View, and Los Altos . The "South Bay" includes all of the rest of the cities in Santa Clara county, centered around San Jose, the largest city in Northern California . It is roughly synonymous with Silicon Valley due to its high concentration of tech companies, although the industry also has a significant presence in the rest of the Bay Area . The "North Bay" includes Marin, Sonoma, Napa, and Solano counties, and is the largest and least populated subregion . The western counties of Marin and Sonoma are encased by the Pacific Ocean on the west and the bay on the east, and are characterized by its mountainous and woody terrain . Sonoma and Napa counties are known internationally for their grape vineyards and wineries, and Solano county to the east, centered around Vallejo, is the fastest growing region in the Bay Area . </P> <P> Although the settlement of the Americas occurred some 20,000 years ago, when the first Asians crossed the Bering Strait into Alaska, the earliest archaeological evidence of human habitation of the Bay Area dates to around 3,000 B.C. along the shores of the bay, with evidence pointing to even earlier settlement in Point Reyes in Marin County . The Miwokan and Costanoan Ohlone people, who were living in the Bay Area at the time of first European contact, were possibly descended from Siberian tribes who arrived at around 1,000 BC by sailing over the Arctic Ocean and following the salmon migration . The Ohlone lived in about forty or so tribes spread throughout the lands adjacent to the San Francisco Bay and as far south as Point Sur in near Monterey Bay . </P>

What part of the bay area is san jose