i was reading about the Vlan and to be more specific the disadvantages of VLAN and the information resource mentioned that Vlan has Device limitations as follows :

Device limitations :

The number of Ethernet addresses that can be supported by each device is 500. This is a distribution of about 20 devices per port on a 25 port switch. In an ideal network situation, there is one device per port, for example, a printer, a workstation, and voice IP phone will require 3 ports. If you wanted to have one VLAN assignment for each port, then the maximum VLANs will equal 25.

so what does that mean "The number of Ethernet addresses that can be supported by each device is 500" ? does this mean that the switch (the 25 ports switch ) can handle up to 500 device that's 20 device per / port (assuming each of these are connected to switch which is in turn connected to the 25 port switch ) ?
or it is just bad example ?

and it follows with another disadvantage of VLANs saying :

Port Constraints:

If a hub or switch is connected to one port, every port on that hub must belong to the same VLAN. Hubs do not have the capability to provide VLANs to individual ports, and VLANs can not be extended beyond the device port even if a switch capable of supporting VLANs is attached.

when it say " every port on that hub must belong to the same VLAN" , as i know hubs are layer 1 devices so they don't understand mac address or deal with Ethernet frame that could had tag field inside of it then they don't understand Vlan , so how is this possible ?

So are these info correct or i have to find another resource ?


  • You really need to explain the device model. There are switches that can handle thousands of MAC addresses, and others, like this, that have a much smaller limitation. Remember that a layer-2 broadcast domain will send traffic all over the broadcast domain. The MAC addresses that a switch sees will end up the the switch MAC address table, and some switches have very little memory for that. – Ron Maupin Jun 9 at 21:51
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    ok , so it depend on the device model , and the above information is not a general case right ? – user69023 Jun 9 at 22:06
  • That iscorrect. There are switches that can have over 10,000 MAC addresses, which is pretty crazy for a broadcast domain. – Ron Maupin Jun 9 at 22:07
  • This sounds like a very cheap consumer grade switch – Ron Trunk Jun 9 at 23:04

Your "device limitations" are specific to that exact device. (500 would be a very tiny desktop grade switch.)

By their very nature, all switches have a limit on the number of MACs they can remember. (MAC table size, CAM table size, etc.) It takes memory to store those tables, and there's a finite amount of memory. Very few switches implement per-port tables. So published limits are generally across all ports.

Many of my smaller Cisco switches (2900 series) support 8192 addresses. (by default, different hardware profiles carve up the memory differently) That's across all the ports, and all VLANs. Internally, the VLAN becomes part of the MAC, but that doesn't fundamentally change anything but it does mean a MAC will consume one table slot per VLAN in which it participates. In effect, if you have 5 hosts in VLAN 1 and 5 in VLAN 2, the switch will need 10 slots even if they are the same 5 hosts.

No one has made hubs for decades, so just forget about them.

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