Significant three bytes of the MAC address) available into which to encode the IP address.Īctually, it is even worse than this.only the lower 23 bits are available for use (for The astute reader may have noticed a problem by now: There are only 24 bits (the least If we ignore the four most significant bits of the multicast IP address (which,Īs noted earlier, will always be "1110"), that leaves 28 bits of IP address information which Traffic, we would ideally like a one-to-one mapping between multicast IP addresses and multicast Since it is obviously infeasible to have an equivalent of ARP when transmitting multicast If you aren't interested and reading this but just need to do a conversion you can skip ahead to the Address Converter Tool. Significant 9 bits of the multicast IP address are quite simply ignored and the remainingĢ3 bits are copied into the lowest 23 bits of the MAC address.Īs a consequence of this, 5 bits of "useful" IP address information being scrapped, there is Non-technical reasons which need not detain us here). Not a one-to-one mapping between multicast IP address and multicast MAC addresses. Rather,Įach multicast MAC address will be shared by 32 IP addresses. This has implications for multicast application design. Figure 2-5 Mapping between an IPv4 multicast address and an IPv4 multicast MAC address The first four bits of an IPv4 multicast address are fixed as 1110, mapping the leftmost 25 bits of a multicast MAC address. As soon as there is something like a router between client and server, ARP will only reveal the MAC address of the router since this is the target for your ethernet packets. This protocol is designed to get the MAC address to a given IP address.
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