TCP/IP Tutorial and Technical Overview - IBM Redbooks

corresponding Ethernet address and sends the packet to the destination. The
destination process informs its network device drivers that it wants to receive
datagrams destined for a given multicast address. The device driver enables
reception of packets for that address.
In contrast to standard IP unicast traffic forwarding, the mapping between the IP
multicast destination address and the data-link address is not done with ARP
(see 3.4, “Address Resolution Protocol (ARP)” on page 119). Instead, a static
mapping has been defined. In an Ethernet network, multicasting is supported if
the high-order octet of the data-link address is 0x'01'. The IANA has reserved the
range 0x’01005E000000' through 0x'01005E7FFFFF' for multicast addresses.
This range provides 23 usable bits. The 32-bit multicast IP address is mapped to
an Ethernet address by placing the low-order 23 bits of the Class D address into
the low-order 23 bits of the IANA reserved address block. Figure 6-1 shows the
mapping of a multicast IP address to the corresponding Ethernet address.
Figure 6-1 Mapping of Class D IP addresses to Ethernet addresses
Because the high-order five bits of the IP multicast group are ignored, 32
different multicast groups are mapped to the same Ethernet address. Because of
this non-unique mapping, filtering by the device driver is required. This is done by
checking the destination address in the IP header before passing the packet to
the IP layer. This ensures the receiving process does not receive spurious
datagrams. There are two additional reasons why filtering might be needed:
► Some LAN adapters are limited to a finite number of concurrent multicast
addresses. When this limit is exceeded, they receive all multicast packets.
► The filters in some LAN adapters use a hash table value rather than the entire
multicast address. If two addresses with the same hash value are used at the
same time, the filter might pass excess packets.
Despite this requirement for software filtering, multicast transmissions still cause
less inefficiencies for hosts not participating in a specific session. In particular,
hosts that are not participating in a host group are not listening for the multicast
Chapter 6. IP multicast 239