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Unformatted text preview: 0) and 18.104.22.168
(hex e0.00 40.20) both map into the Ethernet address 01:00:5e:00:40:20.
Since the mapping is not unique, it implies that the device driver or the IP module in
Figure 12.1 must perform filtering, since the interface card may receive multicast frames
in which the host is really not interested. Also, if the interface card doesn't Figure 12.3 Mapping of a class D IP address into Ethernet multicast address.
provide adequate filtering of multicast frames, the device driver may have to receive all
multicast frames, and perform the filtering itself.
LAN interface cards tend to come in two varieties. One type performs multicast filtering based on the
hash value of the multicast hardware address, which means some unwanted frames can always get
through. The other type has a small, fixed number of multicast addresses to listen for, meaning that when
the host needs to receive more multicast addresses than are supported, the interface must be put into a
"multicast promiscuous" mode. Hence, both types of interfaces still require that the device driver
perform checking that the received frame is really wanted.
Even if the interface performs perfect multicast filtering (based on the 48-bit hardware address), since
the mapping from a class D IP address to a 48-bit hardware address is not one-to-one, filtering is still
Despite this imperfect address mapping and hardware filtering, multicasting is still better than
broadcasting. Multicasting on a single physical network is simple. The sending process specifies a
destination IP address that is a multicast address, the device driver converts this to the
corresponding Ethernet address, and sends it. The receiving processes must notify their IP file:///D|/Documents%20and%20Settings/bigini/Doc...omenet2run/tcpip/tcp-ip-illustrated/broadcas.htm (10 of 12) [12/09/2001 14.47.00] Chapter 12. Broadcasting and Multicasting layers that they want to receive datagrams destined for a given multicast address, and the
device driver must somehow enable...
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