Support for multicasting may exist at the physical and data-link layers. Many shared medium networks, such as Ethernet, support the option of broadcasting and multicasting packets, and corresponding addressing mechanisms. However, processors are often required to perform extra processing when receiving a multicast packet since, for example, many Ethernet controllers only support partial address filtering[57]. This is a result of the fact that Ethernet physically broadcasts all packets and only the address identifies a packet as unicast, multicast or broadcast. While a unicast filter is simple to implement in hardware taking advantage of the factory installed addresses, multicast filters are more difficult to implement because hosts can join and leave multicast groups at will, so they have to be programmable in software. Despite its problems, such hardware support is very important even in an internetwork, since packets that reach the lowest level LANs can be efficiently multicast there using native hardware mechanisms.
When network switches are actually point to point devices, we would like the hardware to automatically recognize multicast addresses as such and transmit multicast packets through multiple links using the same buffers. ATM switch designs[58][59] increasingly support parallel transmission of multicast packets over multiple links in hardware, increasing peak switching speeds. In contrast, in older internetwork designs, switching among lower level network links (that may or may not support multicasting in hardware) was designed for heterogeneity. Therefore, the hardware was optimized for the lowest common denominator, namely unicasting. Although such hardware optimizations were conceived as improvements over the original software switches, they can conflict with the needs of new software switches that support multicasting, thus incurring a speed penalty for multicast transmissions[60]. In this sense, hardware optimizations targeted for unicast in both LANs and WANs, may have to be reexamined and probably augmented in light of the increasing popularity of multicasting. The emphasis placed on high speed network switches to support multicasting[61][62][63][64] could influence more conventional hardware.