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New Member

Maximum subnet size?

How many hosts can be placed on a subnet before broadcasts become a performance issue? I'm looking for a broad estimate here.

My department wants to enable multicast updates to student lab computers from a single multicast server. Currently the lab workstations reside on 5 separate class C subnets. The multicast server must reside on the same subnet as the client computers, so we're looking in to combining these subnets into one larger subnet. There are currently just over 700 lab workstations total.

Is there another way to accomplish this without resorting to placing a multicast server on each of the 5 subnets? I also would like to avoid creating one large broadcast domain. Any suggestions?

New Member

Re: Maximum subnet size?

Before I combined the subnets, I would try to run PIM sparse or PIM Dense on the routers to enable multicasts to cross routers.

To answer your question: it depends.

However, at the beginning of the 100 Mb switch era,

the rule of thumb was 1000 IP users, 500 IPX users

or 250 Appletalk users.

To minimize problems, run WINS on any Microsoft networking to minimize B-node netbios requests, run only multicast routing protocols, try to keep all links to 100 mb, not 10 mb, summarize wan links to avoid topology changes. Also, faster hosts are less affected by broadcasts.

I am sure I forgot something. Hope this helps.



New Member

Re: Maximum subnet size?

Thank you very much Doug, that was helpful.

I was not aware of the PIM feature. I'll have to do some more research to see how to do this appropriately on my 6509.

Thanks again!