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I've seen this on my Windows 7 computer, and searches have turned up others in Vista/7 who have seen the same. However, I've not yet found (or perhaps have just missed) a good explanation for this.

These two addresses are static in my ARP table, but I don't know for what purpose:

224.0.0.22            01-00-5e-00-00-16     static
224.0.0.252           01-00-5e-00-00-fc     static

One website calls these MAC addresses "Internet Reserved by IANA"/"Internet Multicast"/"MPLS Multicast". Still, I don't understand the purpose of these addresses in my table, since they wouldn't match anything on the local subnet.

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What device(s) do those MAC addresses belong to? –  MaQleod Dec 7 '10 at 16:14
    
@MaQleod - If I knew, I wouldn't be asking. coffer.com/mac_find says they're Internet Reserved by IANA/Internet Multicast/MPLS Multicast. I'll update the question. –  Iszi Dec 7 '10 at 16:21
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2 Answers 2

up vote 4 down vote accepted

An extensive explaination on multicast can be found at Firewall.cx. A long excerpt of an even longer article:

A multicast is similar to a broadcast in the sense that its target is a number of machines on a network, but not all. Where a broadcast is directed to all hosts on the network, a multicast is directed to a group of hosts. The hosts can choose whether they wish to participate in the multicast group [..]

As you are aware, each host on an Ethernet network has a unique MAC address, so here's the million dollar question: How do you talk to a group of hosts (our multicast group), where each host has a different MAC address, and at the same time ensure that the other hosts, which are not part of the multicast group, don't process the information?

Hardware/Ethernet Multicasting [..] When the network card picks up a packet which has a destination MAC that matches any of the multicast MAC addresses, it will pass it to the upper layers for further processing. [..] Ethernet uses the low-order bit of the high-order octet to distinguish conventional unicast addresses from multicast addresses. A unicast would have this bit set to ZERO (0), whereas a multicast would be set to ONE (1).

IP Multicast [..] With IP multicasting the hardware multicasting MAC address is mapped to an IP Address. Once Layer 2 (Datalink) picks the multicast packet from the network (because it recognises it, as the destination MAC address is a multicast) it will strip the MAC addresses off and send the rest to the above layer, which is the Network Layer. At that point, the Network Layer needs to be able to understand it's dealing with a multicast, so the IP address is set in a way that allows the computer to see it as a multicast datagram.

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Another good explanation of what these addresses could be used for, but still missing the "how". How did these addresses end up in the table. Is it Windows 7/Vista feature, or some other known software package? –  Iszi Dec 7 '10 at 16:57
    
@Iszi, I assume something did a multicast, leaving the traces in the table? –  Arjan Dec 7 '10 at 17:00
    
As shown in my copy of the table, these are static entries. This does not occur through normal network interactions. –  Iszi Dec 7 '10 at 17:04
    
Moved the "how" of this question to a separate thread: superuser.com/q/219439/50086 –  Iszi Dec 9 '10 at 16:13
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According to Wikipedia:

A multicast address is a logical identifier for a group of hosts in a computer network, that are available to process datagrams or frames intended for a designated network service. [..] The following table is a partial list of well-known IPv4 addresses that are reserved for IP multicasting [..]

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Could you please expand upon this? What would have added these addresses to the table? –  Iszi Dec 7 '10 at 16:27
    
Moved the "how" of this question to a separate thread: superuser.com/q/219439/50086 –  Iszi Dec 9 '10 at 16:12
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