If you have a wireless network that you secure by restricting the MAC addresses, then you will have to enter the MAC address of your Apple iPhone in order to allow it to connect to that network.
To figure out the MAC address of your iPhone, start at the home menu. Select Settings -> General -> About. Scroll down that screen and you’ll see a setting called “Wi-Fi Address”, that is your iPhone MAC address.
Now enter that exact address as an allowed MAC address on your wireless network. The directions on this will vary because every router’s menu and interface is different so consult your router manual if you need help with this.
Once you have done this, go back to your iPhone and go to the Settings menu again. Select Wi-Fi and it should display your network in the list of available networks to connect to. Click the network name to connect.