VLAN port counts

[j.mccartney]

I have four VLAN's and I'm trying to get a countof how many potentially free IP 's have out there now. I had the DNS flushed so that unconnected ports would time out and 3 of the four exceed the total amount that they should be capable of holding.

One VLAN is a /24 but it totals up 257 IP's, another /24 totals 284 and a third with a /25 totals 136 IP's. The only one that makes sense is a fourth VLAN /24 with 149 IP's. Am I missing something?

Thanks.

[Richard Burts]

I do not understand how you gathered your data and wonder if there was a flaw in how the data was generated: is it possible that whatever generated your device counts gave false positives (several devices claiming the same IP address - perhaps DHCP with a very short lease time; or devices with an incorrect mask which got included when they should not have been)

I am also not clear whether your counts are counts of IP addresses or counts of devices. This might make some difference in how to interpret the results.

[j.mccartney]

I had the DNS pools refreshed and the counts came out similar to before. After re-freshing I did a "sh vlan "X" on each switch to get the total number of ports being used per VLAN. Is there a better way? Any help is appreciated.

[Richard Burts]

I am still not quite clear about the DNS pool numbers. But your response is pretty clear about port numbers. When you do show vlan x, it will list all the ports in the vlan which is potentially the number of devices to allocate in the subnet. One thing to be cautious about is the relationship of how many switch ports to how many devices. I am assuming one device per switch port. If the switch port is connected to a hub or to another switch then the number of devices needing IP addresses could be even larger.

But as a starting point I would suggest using the number of ports in vlan x as the number of IP addresses that you would need for that subnet.

a /24 will provide 254 useable IP addresses. If the number of addresses in the subnet is larger than 254 then you will need a subnet larger than /24.

[j.mccartney]

The DHCP clients (PCs) reset their network

settings to uninitiated and request a new DHCP lease from the DHCP server. In effect, all the clients within a pool are booted off the network and have to reacquire a new address as if they never had one. This was how I trying to find out how many IP's were being used in a VLAN to see if I had enough IP's to add more for a particular VLAN. HTH's with what I was trying to accomplish. Thanks for your input.

[Richard Burts · Accepted solution]

Well part of the confusion was that your question mentioned DNS pools when what you meant was DHCP pools.

I would think that counting ports (show vlan x on all the switches) would give you the worst case number for the addresses required. Counting the DHCP pool might be the best case since it is possible that not all PCs would be online at the time that you flush the pools so the number of addresses used in the pool might slightly underestimate the requirements. I suspect that the real answer lies somewhere in between those numbers.