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ARP vs IP:
There is a historical reason for this, as @ronmaupin alludes to.
In small networks, you don't need a layer 3 protocol. All the devices are directly addressable, so layer 2 addresses work fine. As networks got bigger and became interconnected, there was a need to know how to get from one network to another. That is the function of routing, which is done at layer 3.
IP was not the only (or even the most popular) L3 protocol, and Ethernet was not the only layer 2 protocol. That is why there are layers. You can isolate the function of one from the other. It made developing network software easier because you didn't have to have a special version for every type of network.
The layer 3 function doesn't know or care whether you use Ethernet, token ring, PPP, or a dozen other obsolete network protocols. Similarly, the layer 2 protocol doesn't care if you're using IP, IPX, or something else.
The price you pay for layering is that you have to "map" between a 32 bit IP address and a 48 bit MAC address (for Ethernet). That is the purpose of the ARP protocol - to map between layer 2 and layer 3.
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aptitude why to understand why a package was installed or on fedora/centos repoquery --whatrequires what about arch/gentoo?
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Write about tcpwrapper as a tool for network access control
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More on nginx configurations, to enable tls with certbot and http2
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More on RAID and LVM
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consider to move the part on bash scripting in the bash tutorial
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fix fix fix style
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Check where is the formula spazio occupato and so on
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ip netns list/add and man ip-netns
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break entire guide into pieces - ok -