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I have this simple question and really don't know the answer. Does the drawing below show one network or two networks? This is a question about the definition of a network from the OSI / TCP/IP model point of view:

  • From one point of view, those are two L2 networks connected with a bridge.
  • From another point of view, this is one L3 network, that can have a common L3 address space (like 10.1.1.0).

PS

If this question is too dumb, please move it to Superuser.

enter image description here

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+1 for a clear and well formed question. –  Bart De Vos May 27 '12 at 17:37

2 Answers 2

up vote 10 down vote accepted

It's one network. The bridge "bridges" the two physical segments into a single network. A bridge and a switch are (for all intents and purposes) the same thing in that they operate at Layer 2. A switch may also operate at Layer 3 (a L3 switch), but when it's performing L3 operations it's acting as a router not a switch.

Broadcasts to FF:FF:FF:FF:FF:FF (255.255.255.255) are called limited broadcasts and are meant for every node on the same L2 network. An example of this type of broadcasting would be an ARP request.

Broadcasts to the network/subnet broadcast address (192.168.1.255 on the 192.168.1.0/24 network for example) are called directed broadcasts and are meant for every node on the same L3 network. An example of this type of bradcasting would be a NetBIOS Name Query.

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thanks very much. One more question, why do you mention the L3 address for a limited broadcast? (does a limited broadcast need an IP address at all?) –  colemik May 27 '12 at 17:13
    
I'm going to reply to myself: there are basically three types of broadcasts: L2 broadcast (FF:FF:FF:FF:FF:FF), L3 limited broadcast and L3 directed broadcast. If limited broadcast is L3 broadcast, it needs L3 destination data. And IP is L3 protocol. Note also that 255.255.255.255 will anyway be transcribed to L2 broadcast address (i.e. to FF:...:FF if the underlying L2 protocol is Ethernet), bc in order to send the packet over a local network, the packet needs to be wrapped with frame header. And, L2 header needs L2 destination address. –  colemik May 29 '12 at 22:52

1 network, there is a bridge between, which split the collision domains. The bridge splits one big network into two network segments. But it is still one network.

The thing that is confusing is the word "network" it depends on how you define network. Network can be a set of connected computers, but the internet is also a network.

In this case every pc is on the same broadcast domain so you can safely say that it is one network but it exists of network segments.

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thanks for the response. So if I'd configure both 'networks' (your definition) to have one common IP address space(10.1.1.0/24), what would that new structure be called - a 'network' also? –  colemik May 27 '12 at 14:19
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edited it, the correct terminology is network segment but you're still in the same network. –  timmeyh May 27 '12 at 14:22
    
In this case every pc is on the same broadcast domain so you can "safely say that it is one network." - by 'the broadcast domain', do you mean L2 broadcast domain or L3 broadcast domain? (I suspect L3). If yes, then how L2 broadcast domain is called. ;) –  colemik May 27 '12 at 14:32
    
L2 broadcast is just a broadcast to FF:FF:FF:FF:FF:FF don't know the "official" name for it. –  timmeyh May 27 '12 at 14:37

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