broadcast domain

[ilyas001]

hi guys i study networking and i know a broad cast is when you sent a message to all devices at once ok but why single broadcast domain ? i mean a broadcast domain is between only 2 pc's because we use a switch so why name it broadcast because all this terms started colliding in my head (you get it collision domain broadcast domain .... ok i shut up ) 

[brwainer]

A broadcast domain is all the computers, routers, etc that would receive a broadcast message. When a switch receives a broadcast message, it sends that message out every port except the one it was revceived on (any wireless adaptor is considered just another port for switching purposes) So in a very basic network, the broadcast domain would be all the switches that are connected to each other. This can get limited by VLAN, port isolation policies, and other administrative functions of smart or managed switches. The broadcast domain can get extended if you bridge ports together on a computer or router, which normally don't retransmit broadcast packets. 

[ilyas001]

thanks mate  but if we take a switch for exemple it only broadcast at the start but after i knows all the pc's it doesn't need to so why we keep naming it a broadcast domain that's what i don't get ? and why single broadcast domain so it can send only one broadcast at once ? 

[brwainer]

1 minute ago, ilyas001 said:

thanks mate  but if we take a switch for exemple it only broadcast at the start but after i knows all the pc's it doesn't need to so why we keep naming it a broadcast domain that's what i don't get ? and why single broadcast domain so it can send only one broadcast at once ? 

Many many packets are broadcast all the time - DHCP, ARP, discovery protocols to find other computers on the network for things like file and printer sharing, etc. There are a lot of local network protocols that relie on broadcast packets to function.

[ilyas001]

ok i got it now no need for me to complicate it it will always need to broadcast and that's why it's a broadcast domain but why single broadcast domain what single stands for ? 

[brwainer]

Just now, ilyas001 said:

ok i got it now no need for me to complicate it it will always need to broadcast and that's why it's a broadcast domain but why single broadcast domain what single stands for ? 

I'm not familiar with the term "single broadcast domain". I suspect wherever you are reading that term is simply referring to the fact that a switch may have many broadcast domains, based on VLANs and other functions, and they are just saying that the switch will only send out packets on the single domain.

[ilyas001]

21 minutes ago, brwainer said:

I'm not familiar with the term "single broadcast domain". I suspect wherever you are reading that term is simply referring to the fact that a switch may have many broadcast domains, based on VLANs and other functions, and they are just saying that the switch will only send out packets on the single domain.

ok men thanks for your help 

[leadeater]

7 hours ago, brwainer said:

I'm not familiar with the term "single broadcast domain". I suspect wherever you are reading that term is simply referring to the fact that a switch may have many broadcast domains, based on VLANs and other functions, and they are just saying that the switch will only send out packets on the single domain.

I've also not herd the term single broadcast domain, there isn't really a purpose/use for putting single in front of it. A broadcast domain is a broadcast domain simple as that and is a function of layer 2 and/or layer 3 networking.

 

When VLANs are involved you treat each one as their own physical network and draw them out just the same, repeat multiple times for each VLAN. Not that I have ever actually worried about broadcast domains in real life and mapped them out, something you only do when learning.

[leadeater]

7 hours ago, ilyas001 said:

thanks mate  but if we take a switch for exemple it only broadcast at the start but after i knows all the pc's it doesn't need to so why we keep naming it a broadcast domain that's what i don't get ? and why single broadcast domain so it can send only one broadcast at once ? 

I think you are confusing collision domains with broadcast domains when comparing switches to hubs, granted there is conflicting terms used when reading about the functions of these devices.

 

Hubs are a single collision domain, any joining hubs are part of this same collision domain.

Switches have a collision domain per port.

 

Where the confusion typically comes from is a hub is described as broadcasting all frames on every port, this is actually not the same thing as a network broadcast or anything to do with a broadcast domain. It's just effectively the same thing, welcome to the annoying problems of the english language.

 

These are both layer 2 devices and layer 2 devices do not break up broadcast domains, treat VLANs as separate physical switches and it still makes sense. Layer 3 devices break up layer 2 broadcast domains.

 

There are also two kinds of broadcasts, layer 2 and layer 3. A layer 2 broadcast has a destination mac address of FF:FF:FF:FF:FF:FF and a layer 3 broadcast has a destination mac address of FF:FF:FF:FF:FF:FF and a destination IP of the network broadcast address i.e. 192.168.1.255 or 255.255.255.255 (special case). A network broadcast address is when every host bit in a subnet is 1.

 

If you need anything explained in further detail or something clarified let me know, the above is a rough summary of things which typically have page(s) for each item.

 

Edit:

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7 hours ago, ilyas001 said:

switch for exemple it only broadcast at the start but after i knows all the pc's it doesn't need to

When a switch is first turned on they start out in flooding mode and operate much like a hub until the forwarding table has some mac address entries in it, then it will stop flooding and start switching.

Edited August 9, 2016 by leadeater

[ilyas001]

7 hours ago, leadeater said:

I think you are confusing collision domains with broadcast domains when comparing switches to hubs, granted there is conflicting terms used when reading about the functions of these devices.

 

Hubs are a single collision domain, any joining hubs are part of this same collision domain.

Switches have a collision domain per port.

 

Where the confusion typically comes from is a hub is described as broadcasting all frames on every port, this is actually not the same thing as a network broadcast or anything to do with a broadcast domain. It's just effectively the same thing, welcome to the annoying problems of the english language.

 

These are both layer 2 devices and layer 2 devices do not break up broadcast domains, treat VLANs as separate physical switches and it still makes sense. Layer 3 devices break up layer 2 broadcast domains.

 

There are also two kinds of broadcasts, layer 2 and layer 3. A layer 2 broadcast has a destination mac address of FF:FF:FF:FF:FF:FF and a layer 3 broadcast has a destination mac address of FF:FF:FF:FF:FF:FF and a destination IP of the network broadcast address i.e. 192.168.1.255 or 255.255.255.255 (special case). A network broadcast address is when every host bit in a subnet is 1.

 

If you need anything explained in further detail or something clarified let me know, the above is a rough summary of things which typically have page(s) for each item.

 

Edit:

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When a switch is first turned on they start out in flooding mode and operate much like a hub until the forwarding table has some mac address entries in it, then it will stop flooding and start switching.

still don't get it dude look a hub is a layer 1 and i think it have a broadcast domain tooo no ?  just make it simple and clear a broadcast domain means that we will have to use a broadcast message to find the pc we are searching for right ? so even with the mac table we still gonna use broadcast from time to time for example if we add a new objects or change a pc or change an Ethernet card so it's a broadcasting  constantly over the network that's why it's names a broadcast domain but collision is constant it will never effect the other domains that's why it have many this is how i understand it . i should 

[brwainer]

43 minutes ago, ilyas001 said:

still don't get it dude look a hub is a layer 1 and i think it have a broadcast domain tooo no ?  just make it simple and clear a broadcast domain means that we will have to use a broadcast message to find the pc we are searching for right ? so even with the mac table we still gonna use broadcast from time to time for example if we add a new objects or change a pc or change an Ethernet card so it's a broadcasting  constantly over the network that's why it's names a broadcast domain but collision is constant it will never effect the other domains that's why it have many this is how i understand it . i should 

Unless you're going to be dealing with old networks, there is no reason to even be thinking about hubs. You just need to know that they were used early on because they were simpler and therefore cheaper. Gigabit ethernet doesn't even have support for hubs, and at 100Mb it was highly discouraged.

 

A hub never learns MAC addresses. It is more like a splitter than a switch - literally every packet that comes in one port goes out the others. 

[ilyas001]

5 hours ago, brwainer said:

Unless you're going to be dealing with old networks, there is no reason to even be thinking about hubs. You just need to know that they were used early on because they were simpler and therefore cheaper. Gigabit ethernet doesn't even have support for hubs, and at 100Mb it was highly discouraged.

 

A hub never learns MAC addresses. It is more like a splitter than a switch - literally every packet that comes in one port goes out the others. 

ok thanks but did i got it right with the broadcast domain explanation or not ? that's the most important thing here 

[leadeater]

6 hours ago, ilyas001 said:

still don't get it dude look a hub is a layer 1 and i think it have a broadcast domain tooo no ?  just make it simple and clear a broadcast domain means that we will have to use a broadcast message to find the pc we are searching for right ? so even with the mac table we still gonna use broadcast from time to time for example if we add a new objects or change a pc or change an Ethernet card so it's a broadcasting  constantly over the network that's why it's names a broadcast domain but collision is constant it will never effect the other domains that's why it have many this is how i understand it . i should 

 

44 minutes ago, ilyas001 said:

ok thanks but did i got it right with the broadcast domain explanation or not ? that's the most important thing here 

A broadcast domain means when a broadcast is sent every device in the domain will receive it. A broadcast is an actual type of network frame, not everything is a broadcast.

 

A collision domain is when every device will send and receive every frame on the domain, regardless if it is destined for that device. This is why Carrier sense multiple access with collision detection (CSMA/CD) exists, however modern networking with switching and full duplex connections this is essentially no longer relevant.

 

Quote

CSMA/CD was used in now-obsolete shared media Ethernet variants (10BASE5, 10BASE2) and in the early versions of twisted-pair Ethernet which used repeater hubs. Modern Ethernet networks, built with switches and full-duplex connections, no longer need to utilize CSMA/CD because each Ethernet segment, or collision domain, is now isolated. CSMA/CD is still supported for backwards compatibility and for half-duplex connections. IEEE Std 802.3, which defines all Ethernet variants, for historical reasons still bears the title "Carrier sense multiple access with collision detection (CSMA/CD) access method and physical layer specifications".

https://en.wikipedia.org/wiki/Carrier_sense_multiple_access_with_collision_detection#Applications

 

The most important thing to remember is a collision domain and broadcast domain are completely unrelated and have nothing to do with each other. When you are trying to understand them look at them in isolation and don't think about the other in any way.

[ilyas001]

18 minutes ago, leadeater said:

 

A broadcast domain means when a broadcast is sent every device in the domain will receive it. A broadcast is an actual type of network frame, not everything is a broadcast.

 

A collision domain is when every device will send and receive every frame on the domain, regardless if it is destined for that device. This is why Carrier sense multiple access with collision detection (CSMA/CD) exists, however modern networking with switching and full duplex connections this is essentially no longer relevant.

 

https://en.wikipedia.org/wiki/Carrier_sense_multiple_access_with_collision_detection#Applications

 

The most important thing to remember is a collision domain and broadcast domain are completely unrelated and have nothing to do with each other. When you are trying to understand them look at them in isolation and don't think about the other in any way.

i understood you and i know about csma/cd the only thing that i keep in my mind is  just this last question and i'm done . a broadcast in broadcast domain just means sending an information like an upload nothing have to do with the other broadcast right ? this is just what i need to know