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'''Description:''' The link layer, which is the method used to move packets from the network layer on two different hosts, is not really part of the Internet protocol suite, because IP can run over a variety of different link layers. The processes of transmitting packets on a given link layer and receiving packets from a given link layer can be controlled both in the software device driver for the network card, as well as on firmware or specialist chipsets. These will perform data link functions such as adding a packet header to prepare it for transmission, then actually transmit the frame over a physical medium.

For Internet access over a dial-up modem, IP packets are usually transmitted using PPP. For broadband Internet access such as ADSL or cable modems, PPPoE is often used. On a local wired network, Ethernet is usually used, and on local wireless networks, IEEE 802.11 is usually used. For wide-area networks, either PPP over T-carrier or E-carrier lines, Frame relay, ATM, or packet over SONET/SDH (POS) are often used.


'''An example of an attack:'''

Media Access Control (MAC) Address spoofing: 

MAC spoofing attacks involve the use of a known MAC address of another host to attempt to make the target switch forward frames destined for the remote host to the network attacker. By sending a single frame with the other host's source Ethernet address, the network attacker overwrites the CAM table entry so that the switch forwards packets destined for the host to the network attacker. Until the host sends traffic it will not receive any traffic. When the host sends out traffic, the CAM table entry is rewritten once more so that it moves back to the original port.


'''A way to defense:'''

The best way to protect against MAC spoofing is for an intelligent WLAN system to automatically detect MAC spoofing attacks and exclude offending machines from attaching to the WLAN. This is done in several ways:

Detection and Containment - One way to prevent MAC spoofing attacks is to flag any occurrence in which the manufacturer name of a detected WLAN adapter differs from the known OUI (Organizationally Unique Identifier) for that equipment. Once detected, an intelligent WLAN system can prevent the known attacker from connecting to any nearby APs or any APs located throughout the entire WLAN.