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While IP addresses are tied to a specific network, Ethernet addresses are not. Why is this?

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Because IP addresses are reachable from everywhere on the Internet, while Ethernet addresses are confined within the same network.

IP uses routing – specific address prefixes, like 12.34.*.* or 56.7.89.*, are routed towards specific networks that "own" those addresses. These routes are distributed across the entire Internet using BGP. This works because all computers within a network have addresses with the same prefix.

(IP routing is hierarchical – a network can have its own internal routes for longer prefixes. I think some other protocols, like IPX, were also routable but without any hierarchy.)

Meanwhile, Ethernet addresses are assigned pretty much randomly at manufacturing time, and have nothing in common with each other. So Ethernet switches have to "learn" every single address they see on the network.

Most networks are small enough that Ethernet switches can keep track of all computers in the network, but doing the same for the entire Internet would require much more memory and CPU power in the switches than IP routing does. (There are a bit over 500 thousand IPv4 routes, and only the "core" routers have to know all of them – for many routers, a small part is enough. If you had a route for every Ethernet address, I think it'd be at least 8000 thousand...)

So the Ethernet addresses are used only within the same network that the computer is on. (The rest of the Internet doesn't care about them.)

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  • Thank you very much you understand my problem precisely.Thanks Grawity
    – Terminator
    Dec 8, 2014 at 2:39
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If the physical address was "per network", then every device would require configuration to connect to each network, which would render auto-configuration impossible (At least DHCP can send the configuration to the physical address, knowing only one device will accept it; if you had no physical address configured, you could only broadcast new configurations, which runs the risk of duplication)

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