Broadcast and collision Domains

Could someone explain to me what each of these are, I am having difficulty with the subject. I guess I would understand it better if you can tell me why they are needed and when they should be implemented. I would really appreciate it.

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from an on-line dictionary

1 a : complete and absolute ownership of land -- compare EMINENT DOMAIN b : land so owned 2 : a territory over which dominion is exercised 3 : a region distinctively marked by some physical feature 4 : a sphere of knowledge, influence, or activity 5 : the set of elements to which a mathematical or logical variable is limited; specifically : the set on which a function is defined 6 : any of the small randomly oriented regions of uniform magnetization in a ferromagnetic substance 7 : INTEGRAL DOMAIN 8 : the highest taxonomic category in biological classification ranking above the kingdom 9 : any of the three-dimensional subunits of a protein that are formed by the folding of its linear peptide chain and that together make up its tertiary structure 10 : a subdivision of the Internet consisting of computers or sites usually with a common purpose (as providing commercial information) and denoted in Internet addresses by a unique abbreviation (as com or gov); also : DOMAIN NAME

4 (and 3) is the best fit here I think (and of course 10 is about the worst fit).

4 : a sphere of activity

Collision domain The region of a network over which a single collision is detected and propagated. In a modern switched network it is consists simply of a single wire and the two ports at either end of it.

Broadcant domain (typos is us) The region of a network over which a single Layer 2 broadcast frame is propagated. In a modern switched network it consists of all ports and links comprising a single VLAN. (A single subnet).

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A collision domain is basically an area where 2 or more nodes on a network might send a signal at the same time, causing neither side to actually receive that signal. Collision domains are not really a problem with modern equipment, since switches give each node it's own collision domain (and have a ton of other benefits) . However, people that have not upgraded might still have a hub. A hub puts everything attached to it on essentially the same wire, so collisions can occur and screw up your network. Collisions are a layer 1 and 2 issue, electrical signals are the problem, and the CSMA/CD solves it.

Remember that a broadcast is a type of message, broadcast domain is the area that the broadcast signal is allowed to go. Just like your local news doesn't go to Australia. So broadcasts are used whenever the a node wants to send info but doesn't know who should receive it, or when it wants everyone to get a signal. DHCP for instance, the computer sends out a broadcast hoping that someone in the broadcast domain is a dhcp server. Broadcasts don't cross routers (there are exception but you'll learn that later), so all the switches nodes and hubs between routers are in the same broadcast domain (unless there are vlans, but you'll learn about that later too :).

So collisions domains you want to make smaller whenever you can, it helps the network. Broadcast domains vary depending on which clients and server you want to receive broadcasts.

Hope that helps

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