IP sits between the Data Link layer and the Transport layer. One of the goals in the development of IPv6 was to be able to support as many different physical networks as possible and to require no changes in the Transport layer. This approach is called "IP over Everything." To make IP as independent as possible from the Data Link layer, it needs an interface to this layer, which can be Ethernet, ATM, Token Ring, or any other media. The interface needs to be flexible and must be able to adapt to different requirements. For this purpose, features such as Path MTU discovery and fragmentation have been optimized. For UDP and TCP, it should not matter whether IPv4 or IPv6 is used. Obviously, changes are needed whenever IP addresses are used because of the difference in the address format. All these requirements lead to changes built into the IP layer itself. Multicast has been enhanced, and broadcast will not be used with IPv6. This chapter discusses the interface to the Data Link layer.