Single LAN multicast is possible without the need for a multicast router. However, LANs do not understand IP addresses they understand MAC addresses. We need address resolution.
	MAC multicast addresses cannot be hardwired into LAN adaptor cards in the same way as ordinary MAC addresses. They need to be configured at run-time, i.e. the host must tell its LAN adaptor which multicast MAC addresses to listen for. This must be done the first time a process on the host expresses interest in joining a particular IP multicast group. At this point, the host needs to map the IP multicast group address to a MAC multicast address which it can pass to the adaptor. The mapping must be identical in all hosts and in the router since all participants in the group must end up listening to the same MAC multicast address. This could be done through consultation with a server or, perhaps, a broadcast address resolution protocol could be devised. In fact, the decision made was that the mapping should be algorithmic.
	IANA owns a block of Ethernet addresses in the range 00:00:5e:00:00:00 to 00:00:5e:ff:ff:ff and allocates the lower half of these for multicast. The Ethernet convention is that the first byte must be set to 01 to indicate a multicast address. Therefore the range we can use for multicast is 01:00:5e:00:00:00 to 01:00:5e:7f:ff:ff . This means we have 23 bits to play with. These bits are set to the low-order 23 bits of the IP multicast group address to generate the MAC address. So, the address 224.20.5.1, which is e0.14.05.01 in hex, will map to the MAC address 01:00:5e:14:05:01. This is shown in binary below. (We have shown the bit ordering in the conventional way so that 0x01 appears as 00000001. In fact the bits are inserted into the Ethernet frame fields with each byte reversed - so, for example, that the first byte goes out on the wire as 10000000.)
	Now, this is obviously not a 1-1 mapping and it is possible that we end up with two IP multicast groups on a LAN mapped to the same MAC multicast address. This is unfortunate, but not disastrous. It means that a host which has joined the group with address 224.20.5.1 will also receive datagrams intended for (say) 224.148.5.1 and will have to filter these out in software. However, many LAN interface cards do not filter multicast traffic efficiently, so this software filtering will need to be present in any case.