Broadcast domain

Who ever wrote this page is confused about what a "Broadcast Domain" is.

"Broadcast domain" refers to a subnet network, such as 192.168.1.0/24, which has the broadcast address of 192.168.1.255. "Broadcast domain" refers to all that is within broadcast range, or all IP addresses that will receive a broadcast message within the Internet Protocol subnet. That means everything within the 192.168.1.0/24 subnet. Subnetting and Boardcast domains pertain to the OSI Reference Model Layer THREE. The "Broadcast domain" is not in reference to MAC addresses. MAC addresses are OSI Layer TWO. The Broadcast domain is identified by the broadcast address, which in turn is identified by the SUBNET MASK. The broadcast address is an Internet Protocol address. Internet Protocol is OSI Layer THREE. The Broadcast domain is OSI Layer THREE not TWO. If it was layer TWO, it would have a MAC address not a LAYER THREE INTERNET PROTOCOL ADDRESS.

The Subnet is 192.168.1.0/24

The "Network Address" is 192.168.1.0

The "Broadcast Address" is 192.168.1.255.

The "Broadcast Domain" is IP addresses 192.168.1.1 through 191.168.1.254

The "Broadcast Domain" is the range of IP addresses that will receive a broadcast message. It is also the IP address that can be assigned to device nodes from 192.168.1.1 to 192.168.1.154. NOTE THE WORD IP!! Internet Protocol. IP is LAYER THREE not TWO!! This is why the first address and the last address are never assigned to a computer. The first address is the network, or subnet address, and the last address is the broadcast address. The "Broadcast Domain" refers to all nodes that can will receive a "broadcast" message within the subnet.

Subnetting deals with the Internet and the Internet Protocol ( IP ). That means, Broadcast domains are LAYER THREE of the OSI Reference Model, not layer two. Also, do not confuse LAN with Broadcast domains. LANs deal with Ethernet. Often LANs are incorporated into a subnet of the Internet.

Layer THREE -->> IP. IP protocol

Layer TREEE -->> IP subnet 192.168.1.0/24

Layer THEEE -->> subnet mask 255.255.255.0

Layer THREE -->> Broadcast address 192.168.1.255

The OSI Layer THREE Broadcast domain includes all IP from 192.168.1.1 to 254.

ARP maps Layer THREE to Layer TWO -->> Ethernet. 802.3 Protocol.

ARP maps IPv4 addresses to MAC addresses.

For example: ARP might map IPv4 address 192.168.1.1 to MAC address F0:4D:A2:F4:6F:39.

When a broadcast message is sent to the Broadcast Domain via address 192.168.1.255, one IP address that it reaches is IP address 192.168.1.1, which is mapped to MAC address F0:4D:A2:F4:6F:39 via ARP. But Broadcast Domains are established by the SUBNET MASK ADDRESS of the Internet Layer, not the Data-link Layer.

If it is base on an Internet address, it is NOT Layer TWO of the OSI Model.

Broadcast domain is determined by Internet subnetting and the subnet mask.

You guys are getting confused and are mixing up LAN protocols with IP protocols and addressing schemes and you are thinking the are all the same. The Broadcast Domain is not defined by the Local Area Network (LAN). The Boardcast Domain is defined by Internet Protocol and by subnetting and be the subnet mask with determines the Broadcast address and the range of IP addresses that are within the Broadcast Domain.

Another example:

Subnet: 131.247.168.0/23

Network Address: 131.247.168.0

Subnet Mask: 255.255.254.0

Broadcast address: 131.247.169.255

Broadcast Domain: IP Addresses 131.247.168.1 through 131.247.169.254

Another example:

Subnet: 131.247.168.128/25

Network Address: 131.247.168.128

Subnet Mask: 255.255.255.128

Broadcast address: 131.247.168.255

Broadcast Domain: IP Addresses 131.247.168.129 through 131.247.168.254

The REFERENCES below to what a Broadcast Domain is are all wrong. Plus, a LAN segment is and Ethernet segment, which is a patch cable or a 10Base2 or a 10Base5 coaxial cable that connects a node. The cable from a switch to a network interface card is an example of a LAN segment. That cable from a switch to a network interface card on a server on rack can also be an isolated collision domain. Look online and read up on Request for Comments. The is the real stuff!

http://www.faqs.org/rfcs/

Or try the Internet Engineering Task Force at http://www.ietf.org/rfc.html

The stuff below is misleading and is incorrect. It will only confuse you.

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A broadcast domain is a logical division of a computer network, in which all nodes can reach each other by broadcast at the data link layer. A broadcast domain can be within the same LAN segment or it can be bridged to other LAN segments.

In terms of current popular technologies: Any computer connected to the same Ethernet repeater or switch is a member of the same broadcast domain. Further, any computer connected to the same set of inter-connected switches/repeaters is a member of the same broadcast domain. Routers and other higher-layer devices form boundaries between broadcast domains.

This is as compared to a collision domain, which would be all nodes on the same set of inter-connected repeaters, divided by switches and learning bridges. Collision domains are generally smaller than, and contained within, broadcast domains.

While some layer two network devices are able to divide the collision domains, broadcast domains are only divided by layer 3 network devices such as routers or layer 3 switches. Separating VLANs divides broadcast domains as well, but provides no means to network these without layer 3 functionality.

Further explanation

The distinction between broadcast and collision domains comes about because simple Ethernet and similar systems use a shared transmission system. In simple Ethernet (without switches or bridges), data frames are transmitted to all other nodes on a network. Each receiving node checks the destination address of each frame, and simply ignores any frame not addressed to its own MAC.

Switches act as buffers, receiving and analyzing the frames from each connected network segment. Frames destined for nodes connected to the originating segment are not forwarded by the switch. Frames destined for a specific node on a different segment are sent only to that segment. Only broadcast frames are forwarded to all other segments. This reduces unnecessary traffic and collisions.

In such a switched network, transmitted frames may not be received by all other reachable nodes. Nominally, only broadcast frames will be received by all other nodes. Collisions are localized to the network segment they occur on. Thus, the broadcast domain is the entire inter-connected layer two network, and the segments connected to each switch/bridge port are each a collision domain.

Not all network systems or media feature broadcast/collision domains. For example, PPP links.

Broadcast domain control

With a sufficiently sophisticated switch, it is possible to create a network in which the normal notion of a broadcast domain is strictly controlled. One implementation of this concept is termed a "private VLAN". Another implementation is possible with Linux and iptables. One helpful analogy is that by creating multiple VLANs, the number of broadcast domains increases, but the size of each broadcast domain decreases. This is because a virtual LAN (or VLAN) is technically a broadcast domain.

This is achieved by designating one or more "server" or "provider" nodes, either by MAC address or switch port. Broadcast frames are allowed to originate from these sources, and are sent to all other nodes. Broadcast frames from all other sources are directed only to the server/provider nodes. Traffic from other sources not destined to the server/provider nodes ("peer-to-peer" traffic) is blocked.

The result is a network based on a nominally shared transmission system; like Ethernet, but in which "client" nodes cannot communicate with each other, only with the server/provider. A common application is Internet providers. Allowing direct data link layer communication between customer nodes exposes the network to various security attacks, such as ARP spoofing. Controlling the broadcast domain in this fashion provides many of the advantages of a point-to-point network, using commodity broadcast-based hardware.

See also

• Network layer
• Collision domain

References

Broadcast Domain Explained