what is the difference between a broadcast frame and broadcast packet

[Am977]

I'm new to networking and i have confusing between these two 

what i know :

broadcast frame is a layer 3 data unit with destination mac address of all Fs

broadcast packet is layer 2 data unit with destination IP address of the subnet broadcast address  and destination mac address of all F's

a subnet boundary aligns with a broadcast domain boundary

my question :

since  a subnet boundary aligns with a broadcast domain boundary the broadcast frame and the broadcast packet would received by the same devices always so there is no difference between them  

[liviu.gheorghe]

Hello @Am977 ,

A little correction:

• a broadcast frame is a layer 2 data unit with destination mac address of all Fs
• a broadcast packet is layer 3 data unit with destination IP address of the subnet broadcast address

A subnet boundary terminates on a Layer 3 device which also terminates the broadcast domain.

Even though broadcast frames and broadcast packets are received by all hosts in a broadcast domain, they serve different purposes at the layers in which they function. For example a DHCP broadcast from a PC, the broadcast frame will be received by all hosts in the broadcast domain, all hosts will examine the frame and pass it up to Layer 3, but at Layer 3 only the DHCP server will process the IP packet, resulted after discarding the Layer 2 header of the frame, further. All the other PC's in the broadcast domain will discard the packet.

Hope this helps.

[MHM Cisco World]

Broadcast packet can be 

255.255.255.255 <- use in dhcp mostly 

10.0.0.255 (for subnet 10.0.0.0/24) <- called limit broadcast packet 

Above two types of broadcast packets need L2 mac to pass through ethernet' the mac address it use is 

FFFF.FFFF.FFFF and this called broadcast frame.

So broadcast packet is use to point to IP header 

Broadcast frame is use to point to mac l2 header 

MHM

[Joseph W. Doherty]

You're correct that the L3 IP broadcast uses a L2 Ethernet frame which is sent to all nodes within the L2 broadcast segment but L3 broadcasts can be routed.

For instance if we're in subnet 192.168.1.0/24 I can ping 192.168.100.255/24.  The latter will use unicast MACs until if reaches the destination subnet.

Or, in theory, we could ping an entire IP network using an IP of 255.255.255.255, but RFC 919 suggests blocking that (due to scalability issues).  So, using the global IP broadcast we can broadcast to the connected L2 domain without needing to know what the connected subnet is.

Here's a riddle, if a L2 segment hosts multiple subnets, for example 192.168.1.0/24 and 192.168.2.0/24 does it matter whether we use an 255.255.255.255 or 192.168.1(or 2).255 destination IP as both will generate the same broadcast MAC?

Riddle hint:

Spoiler

Similar issue with IP multicast and Ethernet multicast.

Riddle answer:

Spoiler

The NIC will accept any Ethernet broadcast frame to further examine whether the host has any interest in it.

If the IP packet has a subnet broadcast IP that's not for the host's subnet, the network stack can stop further analysis of the packet's contents for relevance.

[M02@rt37]

Hello @Am977 

While it's true that both broadcast frames and broadcast packets are received by devices within the same broadcast domain, there are distinctions between the two.

A broadcast frame is a Layer 2 data unit with a destination MAC address of all Fs, and it's typically used to communicate within a local network segment. On the other hand, a broadcast packet is a Layer 3 data unit with a destination IP address of the subnet's broadcast address, which is used to communicate with all devices within the subnet. While they both serve the purpose of reaching all devices within the broadcast domain, the broadcast frame operates at the data link layer (Layer 2) and the broadcast packet operates at the network layer (Layer 3), each serving different functions within the network architecture...

Well-known protocol that utilizes packet broadcast at the network layer is the Address Resolution Protocol (ARP). ARP is used to map IP addresses to MAC addresses within a local network segment. When a device needs to communicate with another device on the same subnet, it sends out an ARP broadcast packet to discover the MAC address associated with the destination IP address. This ARP broadcast packet is received by all devices within the same broadcast domain, allowing the device with the matching IP address to respond with its MAC address.

An example of a protocol that uses frame broadcast at the data link layer (Layer 2) is the Dynamic Host Configuration Protocol (DHCP). DHCP is used to automatically assign IP addresses and other network configuration parameters to devices on a network. When a device initially connects to a network and needs an IP address, it sends out a DHCP discover message as a broadcast frame. This broadcast frame is received by all devices within the same broadcast domain, allowing DHCP servers to respond with IP address lease offers.