What does WEP do? The keystream for WEP is RC4(v,k), which depends only on v and k. k is a fixed shared secret, that changes rarely if ever In fact, in many setups, every user shares the same k So the keystream depends only on v If two packets ever get transmitted with the same value of v, you reuse the keystream, which is bad. Since v gets transmitted in the clear for each packet, the adversary can even easily tell when a value of v is reused (a "collision"). How many possible values of v are there? v only occupies 24 bits of the header, so at most there are 2^24, or about 16 million possible values of v. After 16 million packets, you have to repeat one! Oops.