How can I estimate ethernet performance?

I need to think about performance limitations of 100 mbps ethernet (including scenarios with up to ~100 endpoints on the same subnet) and I'm wondering how best to go about estimating the capacity of the network. Are there any rules of thumb for this?

The reason I ask is that I am working on some back-of-the-envelope level calculations about performance limitations, so it doesn't need to be incredibly accurate. I just haven't been through this exercise before and was hoping to gain some insight from those who have. Mark Brackett's answer (as of 1/26) is along the lines of what I am looking for.

From stackoverflow

• Opnet creates software for simulating network performance. I once used Opnet IT Guru Academic edition. Maybe this application or some other software from opnet may be of some help.

• 100 endpoints are not suppose to be an issue. If the network is properly configured (nothing special) the only issue is the bandwidth. Fast Ethernet (100 mbps) should be able to transfer almost 10Mb (bytes) per second. It is able to transfer it to one client or to many. If you are using hubs instead of switches. And if you are using half-duplex instead of full-duplex. Then you should change that( this is the rule of thumb).

• If you're using switches (and, honestly, who isn't these days) - then I've found 80% of capacity a reasonable estimate. Usually, it's really about 90% because of TCP overhead - but 80% accounts for occasional retransmits.

  If it's a single collision domain (hubs), then you'd probably be around 30% with moderate activity on those 100 nodes. But, it'd be pretty variable based on the traffic generated. And anyone putting 100 nodes in a single CD these days would no doubt be shot - so I don't think you'll actually run into those IRL.

  Edit: Note that these numbers are for a relatively healthy network - one that is generally defined as working. Extremely excessive broadcasts or other anomalous traffic patterns have been known to bring a network to it's knees.

  Luke : What size switches are you assuming? 16 port?

  Mark Brackett : @Luke - Doesn't matter. Any half decent switch will scale the switching fabric capacity linearly as the # of ports go up. Unless all 100 nodes are running full out, the switch will not be the bottleneck.

• Use WANem

  WANem is a Wide Area Network Emulator, meant to provide a real experience of a Wide Area Network/Internet, during application development / testing over a LAN environment.

  You can simulate any network scenario using it and then test your application's behaviour using it. It is open-source and is available with sourceforge.

  Link : WANem - The Wide Area Network emulator