I just spoofed my MAC at my school and connected to router. I got past the firewall, but now I don't have internet access. I am using windows 7.
closed as off topic by Jeff Ferland♦ May 21 '13 at 20:17
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If you've spoofed someone else's MAC address and they're on the network with you chances are you've received the same IP address and are causing enough collisions that the all transferred frames are being dropped or are timing out.
Most routers, bridges, switches, and other network devices all expect to send and receive packets in an order. When you jump in the middle and start trying to receive someone else's packets chaos ensues.
From the sounds of it, trying to change your MAC address on the network to bypass filtering or create anonymity is definitely something the Administrator who is responsible for the network is on the look-out for. If an exploit happens on the network, the Administrator is responsible and will be involved in providing all of the necessary records for the connections being made to whoever is involved in the investigation. He will in fact have to spend some time gathering this information in the event of an issue, so since he's likely a busy person already he would have systems in place to reduce this risk. Some localities require that all traffic be documented.
If you're using a random MAC address to try and change the identity of your machine then it's probably in the best interest of the IT guy that he does block anything that you could do on the network because from a digital paper-trail perspective it's nearly impossible for him to track you through his normal means via connection logs in the router. That being said he can use an app like Wireshark and capture the network frames you're transmitting over the network and make a match for user accounts and accesses so he can then determine that both MAC addresses are likely the same person if the same user account is used on the same website.
Additionally a lot of the schools (Universities) that I've been at will have a network that uses something like WPA2-Enterprise for their WiFi access which in some cases will make it look like you've connected even though you haven't provided all of the proper credentials (it can sometimes look like an open network). School networks (I'm assuming college or high school) will provide you with a login where you can connect with your account to your appropriate shares and they can monitor your activities as required by law.