ARP (Address Resolution Protocol) explained

Lessons Discussion
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Hello Willy

No problem, no reason to apologize (French is a beautiful language, I wish I could speak it fluently!)

So let’s take a look at your example. Just to clarify, I think you have a typo in what you wrote, so let me write it out again. If we have 192.168.1.0/24, then we have the following addresses:

  1. 192.168.1.0 is the network address
  2. 192.168.1.1 to 192.168.1.254 are the host addresses
  3. 192.168.1.255 is the broadcast address.

Now, if you want to send a packet to all the hosts in the network, then you would place the 192.168.1.255 address in the header of the IP packet. The result is that all the hosts will receive such a packet. But how does that work? Because the 192.168.1.255 address is on Layer 3 of the OSI model. What happens when this is encapsulated into an Ethernet frame? Well, that’s where the FF:FF:FF:FF:FF:FF address comes in.

So the host which is sending this broadcast address places the destination IP of 192.168.1.255 into the IP header. This is then encapsulated into an Ethernet frame. From this IP address, we must find the destination MAC address to populate the frame header. The host knows that the IP address is the broadcast address, so the MAC address that must be used in the frame must also be a broadcast address. FF:FF:FF:FF:FF:FF is the broadcast MAC address that must be used. When a switch receives such a packet, it will forward it out of all it’s ports of the same VLAN, thus sending it to all hosts in the subnet.

Broadcast IP addresses are used by features such as DHCP and other mechanisms that allow a network to function. When the broadcast IP is used, the resulting destination MAC used in the encapsulating frame, FF:FF:FF:FF:FF:FF.

Now in the case of an ARP request, remember that ARP is a Layer 2 protocol, and it does not use IP addressing. So in such a case, a broadcast IP address would never be used. The ARP protocol will only use MAC addressing to get its messages to their intended recipients. So an ARP request, because it is always broadcast to all the hosts in the network segment, will use FF:FF:FF:FF:FF:FF as the destination MAC. The IP protocol is not involved here at all.

I hope this has been helpful!

Laz

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