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Dynamic WEP is a change to Wired Equivalent Privacy (WEP) that changes WEP keys dynamically, a change that made it into WPA's TKIP system. Dynamic WEP was likely developed as a response to WEP's security flaws, that can lead to a wireless network being compromised in a matter of minutes. WEP2 and WEPplus are two other extensions on top of WEP, which made changes (some later kept with WPA) that tried to address the security concerns.
More information: Wired Equivalent Privacy (WEP)