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The Layer 2 broadcast is the destination address of the Layer 2 package is FF-FF-FF-FF-FF-FF.

I have a question about the broadcast MAC address.

you see the ARP request, it send the destination MAC address to the broadcast.

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I have questions about it:

  1. Who the actual star of the broadcast MAC address? is it a router ? or a switch?
  2. You see in the LAN, the ARP request send 6 times, I opened one(192.168.1.165). Why the broadcast only send 6 times rather than 256 times(/24)?

EDIT-01

Thanks for response, and you see literally "everyone on this broadcast domain", if this is a /8 segment, whether it will literally so many addresses? or it will only traversal the addresses in the LAN?

I mean if there only 10.10.10.1/8, 10.10.10.2/8, 10.10.10.5/8 in the LAN, if send data to ff:ff:ff:ff:ff:ff, will it traversal the other two addresses, or from the 10.10.10.1 to 10.255.255.255?

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  • It may help you to remember that a MAC broadcast is a layer 2 function. Every host in the layer 2 domain will receive the broadcast. It does not matter what IP address they are configured with.
    – Ron Trunk
    Sep 4, 2019 at 12:00
  • Do not mix up the network layers. Sending a layer-2 frame to the layer-2 broadcast address will deliver the frame to every host in the broadcast domain. If the layer-3 address is a unicast address, then every host except the one with that address will drop it. Broadcasting is wasteful of host resources and network bandwidth, so it was removed from IP with IPv6.
    – Ron Maupin ♦
    Sep 4, 2019 at 12:01
  • it's "everyone on this broadcast domain". The expansion of the broadcast domain is a question of Layer 2 design (as in: how large is your switching domain, and how far does the given VLAN stretch?), largely unrelated to the IP subnet you chose for it. Even if you deployed the IP subnet for the given broadcast domain as 10.0.0.0 /8 (not the best of ideas, anyway), the underlying Layer 2 infrastructure will at most have a few hundred ports. So your most certainly hierarchical switching infrastructure will only have to create a total of a few 100 copies of that broadcast frame.
    – Marc 'netztier' Luethi
    Sep 4, 2019 at 14:04
  • ... but these copies are not sent to hosts individually. In case of an ARP broadcast, MAC DstAddr remains ff:ff:ff:ff:ff:ff, IP DstAddr remains 255.255.255.255. The broadcast frame just gets flooded out of all ports (read up about flood & learn behaviour of switches). Other protocols than ARP talk to the local subnet broadcast address (the last address of the Subnet), in such a case, the MAC DstAddr is ff:ff:ff:ff:ff:ff, the IP DstAddr is 192.168.1.255, for example.
    – Marc 'netztier' Luethi
    Sep 4, 2019 at 14:41

2 Answers 2

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ARP is used to resolve an IP address to the required MAC address for L2 transport. Since the destination MAC is yet unknown it is sent as L2 broadcast.

If the ARP request fails (timeout) most hosts repeat it once or twice. It is possible and quite common that the higher-layer protocol retries as well, repeating the resulting 1-3 ARP requests multiple times as well.

Since ARP requests are broadcast all that is completely independent from the subnet size.

The (limited) broadcast reaches every node in the local L2 segment - it's simply flooded by each switch to all of its port (but the one it was received on). This is true even when there are multiple IP subnets within that L2 same segment.

[Re: EDIT-01]: You're mixing L2 and L3 - IP/L3 has no broadcast mechanism of its own. It can only use a broadcast from the lower L2 layer, if present. Accordingly, it tells Ethernet to send a broadcast that the Ethernet switches propagate to all L2 nodes - that propagation is completely independent of IP.

That broadcast identifies itself as ARP packet by Ethertype. It is processed by those end nodes that implement ARP, and dropped by the others. The local IP stacks then parse the packet and the one node that is addressed sends a response.

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  • is the router host the broadcast address?
    – 244boy
    Sep 4, 2019 at 8:09
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    @244boy no, not the router. The destination address ff:ff:ff:ff:ff:ff is literally "everyone on this broadcast domain". An ARP broadcast is like shouting into the LAN _"Hey everybody on this subnet, I need to talk to IP address ip.ip.ip.ip and I want to know the MAC address of the system with that MAC address!" and usually, the system having that IP address will respond. Switches will copy and send this broadcast out of all ports of the (V)LAN, to make sure that the broadcast reaches all end systems.
    – Marc 'netztier' Luethi
    Sep 4, 2019 at 11:28
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Regarding the additional questions under "EDIT-01":

Thanks for response, and you see literally "everyone on this broadcast domain", if this is a /8 segment, whether it will literally so many addresses? or it will only traversal the addresses in the LAN?

Yes, everyone on this broadcast domain. This is at layer 2, so it doesn't care whether the IP address are in a /8 or not.

I mean if there only 10.10.10.1/8, 10.10.10.2/8, 10.10.10.5/8 in the LAN, if send data to ff:ff:ff:ff:ff:ff, will it traversal the other two addresses, or from the 10.10.10.1 to 10.255.255.255?

Not sure what you are getting at here, but if you mean there are 3 devices in the LAN, they are in the same broadcast domain, so they will all receive the frame sent to the broadcast address.

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