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Handling MAC Addresses
If a networked L2 device such as a bridge or LAN switch
could contain a list of all known MAC addresses, then the network
node could function in much the same way as a router, forwarding frames
instead of packets hop-by-hop through the network from source LAN
to destination LAN. However, the MAC address is much larger than the
IPv4 address currently used on the Internet backbone (48 bits compared
to the 32 bits of IPv4). This poses problems. Also, because the MAC
address has no “network organization” like the IPv4 or
IPv6 address, an L2 network node must potentially store every conceivable
MAC address in memory for next-hop table lookups. Instead of tables
of about 125,000 entries, every L2 network node would have to store
millions of entries (for example, 24 bits, the potential NIC production
from one Ethernet vendor, would require a table
of more than 16 million entries).
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