how many hosts can be supported at best in one vlan?

wang-yang · ‎03-13-2003

thisisshanky · ‎03-14-2003

There isnt an exact figure you can say as to how many hosts canbe supported in one vlan. It depends again on type of applications used by each host ( say if the apps are very chatty- broadcasts are more in the vlan, and hence more traffic. Also it depends on the ip add. scheme you use. Its recommended to use one single ip subnet per vlan. If you use a class C network, you are limited to 254 hosts.

Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus

jsalminen · ‎03-14-2003

In the CCDA course they recommend no more than 500 nodes in a single subnet/network. That is only possible by using class A or B networks or subnets.

efrahim · ‎03-20-2003

Yes you can have as many you want but usually the best practice is one class c subnet means 254 host.

wolfrikk · ‎03-21-2003

You can have as many as you want, but you are bound by the Protocol you network is using. IP's listed limit of nodes is 500, so you should push that Limit.

j.lipsett · ‎04-02-2003

I have a network that currently is split into 2 subnets/vlans of 1024 users. The switch used is a 3com 6012 corebuilder. We are refreshing to cisco switches.

will they handle vlans this size?

John lipsett CCNP

konigl · ‎04-02-2003

Yes. Cisco switches can handle this. Just make sure the ones you use can handle 2 x 1024 = 2048 MAC addresses at a minimum, and you should be fine. (That should be just about Cisco's whole L3/multilayer switch product line, I think.) The LANPlex 6000/CoreBuilder 6012 was a remarkable box in its day; I see Cisco's Catalyst 6500 series in that role today.

The arbitrary limits of 500 or 254 per VLAN mentioned here are design recommendations, based on considerations such as relative amounts of broadcast traffic; the number and kinds of protocols in use on that VLAN; or the number of host machines, computers, or printers (think anything with a MAC address) that the switches can remember.

In theory, you could set up a VLAN with an IP class A network address (e.g. 10.0.0.0/8), no subnetting, and put 16,777,214 hosts on it. That assumes the switches would have enough RAM to remember all those MAC addresses. As a practical matter, most switches don't have that kind of memory. And even if they did, the amount of broadcast traffic that would provide "background noise" on such a hypothetical network would probably keep the attached machines too busy to do any useful work.

I think I remember reading somewhere that Cisco's recommendations are 500 nodes max. per VLAN for IP only, 300 nodes if a mix of IP and IPX, 200 max. if IP and a mix of anything else (e.g. AppleTalk, DEC LAT, NetBEUI etc.).

I have set up IP-only networks using Class B addressing, no subnetting, with over 1000 nodes, that have worked fine. And I have seen multiprotocol VLANs (IP, IPX, and AppleTalk) with more than 500 nodes per VLAN, also working fine.

The issue with how many MAC addresses a switch remembers is this: if the switch doesn't know where to find a particular MAC address, it has to flood traffic for it to all active ports in the hopes of reaching it. If there are more MACs on the VLAN/broadcast domain than what the switch can hold, then addresses churn in and out of the MAC address tables, and needless amounts of broadcast-type network traffic result as the switches have to keep relearning where certain MAC addresses live. The solution to this is to either upgrade the memory of the switch, or replace it with one that's more capable, or limit the number of MACs contained in the broadcast domain and connect those broadcast domains with routers or Layer 3 switches.

Hope this helps.