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    Networking at Layer 2: Challenges of Ethernet MAC Addresses

    If a networked Layer 2 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 Layer 2 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 Layer 2 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).

    Modified: 2017-08-31