AUTH frame

Open authentication (null authentication algorithm) - if encryption is not enabled in the network, any device that knows the SSID can gain access to the network. Otherwise if a device does not have the correct WEP key (and its authentication is successful), the device will be unable to transmit data through the AP.

Shared key authentication - the client and AP have to use a static shared WEP key. After the client sends an authentication request to the AP, the AP responds with an authentication response containing challenge text. The client encrypts the challenge text with its WEP key and replies with a subsequent authentication request. If the AP can decrypt this request and retrieve the original challenge text, it grants the client access.

MAC address authentication - an additional mechanism to the open shared key authentications that reduces the likelihood of unauthorized devices accessing the network