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New Member

What is broadcast address

What is broadcast address

2 ACCEPTED SOLUTIONS

Accepted Solutions
New Member

 hi chethan,     its 255.255

 

hi chethan,

     its 255.255.255.255

Cisco Employee

Hi,While correct answers have

Hi,

While correct answers have already been posted here, let me broaden the scope a little to provide a context.

In data communication, there is often a need to send a datagram to all stations connected to the same medium, or the same link, without even knowing their own addresses. It is like shouting aloud in a room to speak to all present persons at once, without knowing their names. This is broadcasting. We use it often, not just if intentionally want a message to be delivered and processed by all other stations, but also in times when we want the message to be processed only by a single specific device - but we do not know the address of that device, so the only solution is to send the message to everyone and hope that the specific device receives the broadcast as well. Think of DHCP, for example: a client that has just started does not know the address of the DHCP server. The only way of contacting it is to send the DHCP datagrams as broadcasts, hoping that apart from other clients that will receive these DHCP messages (and ignore them), they will also be received by a DHCP server who will then respond.

Because datagrams commonly used in our networks, either frames or packets, have only a single "Destination Address" field, you cannot list all possible recipients in that field of a single frame to make sure that everyone receives it. Instead, a specific pre-defined address is used in place of the destination address field, indicating that this is a broadcast datagram intended to be received and processed by everyone.

  • On Ethernet, the broadcast MAC address is FF:FF:FF:FF:FF:FF
  • In IPv4, a so-called limited broadcast address is 255.255.255.255. Packets destined to this address are never routed through a router.
  • In IPv4, each network and subnet has its specific broadcast address that is also called a directed broadcast. For example, the 192.168.1.0/24 network has the broadcast address of 192.168.1.255. The 10.0.8.64/29 network has the broadcast address of 10.0.8.71. These broadcast addresses are called directed broadcasts because they can actually be used to send broadcasts from a different source than the network itself: From any location, I can send packets destined to 192.168.1.255 and they will initially be routed normally just like any other IP packets but when they reach a router on which a network is directly attached for which the 192.168.1.255 would be a broadcast address, these packet might be forwarded into that network as broadcasts.

There is a relation between broadcast MAC and IP addresses. As you know, when unicast IP packets are sent over an Ethernet network, they are encapsulated into Ethernet frames and the destination MAC is resolved using the ARP protocol. With a packet destined to a broadcast IP address, there is an exception: the IP stack knows immediately that the packet shall be encapsulated into frames with the destination MAC of FF:FF:FF:FF:FF:FF.

It is worth noting that the broadcasting is primarily a function of Layer2. It is the Ethernet that allows sending a single datagram and have it delivered to all stations within an Ethernet domain (LAN or VLAN). You could have your IPv4 driver generate broadcast packets but they must still be encapsulated into Layer2 frames, and if the particular Layer2 technology does not support broadcasts on its own, you simply don't have it. The only thing you can do is to send multiple Layer2 frames carrying the same broadcast IP packet, with the Layer2 frames being addressed to each connected station in turn. You would emulate the broadcast in this way, but it is not a true broadcast.

My two cents...

Best regards,
Peter

5 REPLIES
New Member

 hi chethan,     its 255.255

 

hi chethan,

     its 255.255.255.255

New Member

255.255.255.255

255.255.255.255

Cisco Employee

Hi,While correct answers have

Hi,

While correct answers have already been posted here, let me broaden the scope a little to provide a context.

In data communication, there is often a need to send a datagram to all stations connected to the same medium, or the same link, without even knowing their own addresses. It is like shouting aloud in a room to speak to all present persons at once, without knowing their names. This is broadcasting. We use it often, not just if intentionally want a message to be delivered and processed by all other stations, but also in times when we want the message to be processed only by a single specific device - but we do not know the address of that device, so the only solution is to send the message to everyone and hope that the specific device receives the broadcast as well. Think of DHCP, for example: a client that has just started does not know the address of the DHCP server. The only way of contacting it is to send the DHCP datagrams as broadcasts, hoping that apart from other clients that will receive these DHCP messages (and ignore them), they will also be received by a DHCP server who will then respond.

Because datagrams commonly used in our networks, either frames or packets, have only a single "Destination Address" field, you cannot list all possible recipients in that field of a single frame to make sure that everyone receives it. Instead, a specific pre-defined address is used in place of the destination address field, indicating that this is a broadcast datagram intended to be received and processed by everyone.

  • On Ethernet, the broadcast MAC address is FF:FF:FF:FF:FF:FF
  • In IPv4, a so-called limited broadcast address is 255.255.255.255. Packets destined to this address are never routed through a router.
  • In IPv4, each network and subnet has its specific broadcast address that is also called a directed broadcast. For example, the 192.168.1.0/24 network has the broadcast address of 192.168.1.255. The 10.0.8.64/29 network has the broadcast address of 10.0.8.71. These broadcast addresses are called directed broadcasts because they can actually be used to send broadcasts from a different source than the network itself: From any location, I can send packets destined to 192.168.1.255 and they will initially be routed normally just like any other IP packets but when they reach a router on which a network is directly attached for which the 192.168.1.255 would be a broadcast address, these packet might be forwarded into that network as broadcasts.

There is a relation between broadcast MAC and IP addresses. As you know, when unicast IP packets are sent over an Ethernet network, they are encapsulated into Ethernet frames and the destination MAC is resolved using the ARP protocol. With a packet destined to a broadcast IP address, there is an exception: the IP stack knows immediately that the packet shall be encapsulated into frames with the destination MAC of FF:FF:FF:FF:FF:FF.

It is worth noting that the broadcasting is primarily a function of Layer2. It is the Ethernet that allows sending a single datagram and have it delivered to all stations within an Ethernet domain (LAN or VLAN). You could have your IPv4 driver generate broadcast packets but they must still be encapsulated into Layer2 frames, and if the particular Layer2 technology does not support broadcasts on its own, you simply don't have it. The only thing you can do is to send multiple Layer2 frames carrying the same broadcast IP packet, with the Layer2 frames being addressed to each connected station in turn. You would emulate the broadcast in this way, but it is not a true broadcast.

My two cents...

Best regards,
Peter

New Member

Thanks Peter for detailed

Thanks Peter for detailed response. It is helpful

New Member

Hi Chetan,Please find my

Hi Chetan,

Please find my below answer.

A broadcast address is a special Internet Protocol (IP) address used to transmit messages and data packets to network systems. Network administrators verify successful data packet transmission via broadcast addresses.
Dynamic Host Configuration Protocol (DHCP) and Bootstrap Protocol (BOOTP) clients use broadcast IP addresses to locate and transmit respective server requests.

An address that enables transmission to every node in a local network. The address is the highest numeric value of the address format being used. An Ethernet broadcast address is all binary 1's. An IP broadcast address is the highest number in its class; for example, the broadcast address of a Class C 192.168.16.0 network is 192.168.16.255.
 

 

 

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