Flood attacks overwhelm targets with traffic. Types: SYN flood (exhausts TCP state), ICMP flood (ping flood), UDP flood (stateless), HTTP flood (Layer 7 — hardest to mitigate).
SYN flood defense: SYN cookies (validates TCP handshake without state allocation). HTTP flood requires Layer 7 analysis — CAPTCHA, rate limiting, behavioral analysis. CDN absorbs volumetric floods.