Fastnet can refer to:

• Fastnet International Schools Regatta, a regatta held in County Cork, Ireland
• Fastnet Line, a passenger ferry service operating between Wales and Ireland
• Fastnet (netball), a variation of the rules of netball used primarily in the World Netball Series
• Fastnet Race, one of the four or so most prestigious ocean races in competitive sailing
• Fastnet Rock, a small clay-slate island with quartz veins and the most southerly point of Ireland
• A sea area, named for the rock, used in the British Shipping Forecast
• Fastnet Rock (horse), an Australian thoroughbred racehorse stallion
• FastNet, the business name of South African communication service provider Swiftnet Pty Ltd, a subsidiary of Telkom

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