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Address Resolution Through Direct Mapping
(Page 2 of 3)
Direct Mapping Not Possible With Large Hardware Addresses
Unfortunately, direct mapping only
works when it is possible to express the data link layer address as
a function of the network layer address. Consider instead the same IP
address, 222.101.33.29, running on an Ethernet network. Here, the data
link layer addresses are “hard-wired” into the hardware itself
(they can sometimes be overridden but usually this is not done). More
importantly, the MAC address is 48 bits wide, not 8. This means the
layer two address is bigger than the layer three address, and there
is no way to do direct mapping, as Figure 46
illustrates.
Figure 46: Address Resolution Problems With Large Hardware Address Size When the layer two address is larger in size than the layer three address, it is not possible to define a mapping between them that can be used for address resolution.
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 Note: In the case where the hardware address size exceeds the network layer address size, we could do a “partial mapping”. For example, we could use the IP address to get part of the MAC address and hope we don't have duplication in the bits we didn't use. This method is not well-suited to regular transmissions, but is in fact used for resolving multicast addresses in IPv4 to Ethernet addresses.
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In general, then, direct mapping
is not possible when the layer three address is smaller than the layer
two address. Consider that Ethernet is the most popular technology at
layer two and uses a 48-bit
address, and IP is the most popular technology
at layer three and uses a 32-bit address. This is one reason why direct
mapping is a technique that is not only not widely used, but that most
people don't know about!
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