How do I determine what device is utilizing the majority of the bandwidth on a network?

How do I determine what device is utilizing the majority of the bandwidth on a network?

Problem:  Recently our phone system and internet has been dropping intermentently.  I contact our ISP and they ran some test and they believe that everything is good on there end.  They believe that there is a device or application that is consuming a majority fo the bandwidth and is causing our network problems.  They also mentioned that maybe a device is doing some kind of broadcasting or maybe someone is streaming music or videos.

How do I determine what is causing the problem?
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How many computers do you have in your network ? You could use a network monitoring software like Netlimiter. if you install that software on pc's you can *monitor and/or limit* network traffic for each individual application. You can use the trail version for 30 days.  
cmleavittAuthor Commented:
I have approximately 75 computers.
It's possible that one computer or more to have spyware installed and perform DOS attacks. What I suggest is to take off parts of the network to see if the traffic is decreasing. Do you have switches with management  (ex. cisco)?
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cmleavittAuthor Commented:
I'm not exactly sure what you mean by taking parts off the network?  We do have Cisco switch with managment.  But I have no experience with them.

Just a little background on my IT skills.  I recently moved into a roll as a Network Administrator.  Previously most of my work has been Desktop Support.  So.. I'm learning as go.

Would this cause the problem.... about 2 months ago a installed a new Cisco switch with managment but I did not do any configurations to it.  I just connected it with another Cisco switch via a patch cable because I a ran out of available ports to plug a Server to.

If you have your clients connected to CISCO switch you can login on the sw and use this command
 Sw# show ip int status.
 This will display a list with all SW interfaces, then look for those connected and use the second command.
SW# sh ip interface <interface name (ex. Fa0/0)>  | i rate
Sw# show int Fa0/23 | i rate (Ex)
  Queueing strategy: fifo
  30 second input rate 6189000 bits/sec, 529 packets/sec
  30 second output rate 188000 bits/sec, 254 packets/sec
 input > upload from client
 output> download from client

 By taking off parts of the network I meant to phisically disconnect them and see if you are still using your bandwidth at maximum.
  If you can manage to connect to your cisco sw's you can see each pc individual traffic on that port.  

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I would try using either Ntop or Wireshark to see if you can determine the IP address of the device that is generating the most traffic.  Check out:

Ntop is a network traffic probe and Wireshark is a packet sniffer.  The important thing to know is that to see all of the traffic on the network, you will either have to setup a monitoring or span session on your Cisco switch, or you will need a true dumb hub to mirror all the traffic to the machine you have either of the above apps installed on and then you will see all the network traffic.
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Network Analysis

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