Know how many device in subnet

[Leftz]

Hi there is subnet 10.10.10.0/24 at a switch. I want to know how many devices in this subnet. In addition to third party app, is there some cisco command that can be used at the switch to show how many devices in this subnet? Thanks

[balaji.bandi]

You can have 254 devices maximum in this subnet 1 used for Gateway, 253 can be used for devices

On the Device show ip arp  (list of the device live in that subnet)

[Georg Pauwen]

Hello,

you probably need some sort of script that concatenates various interface information. The command:

show interfaces switchport | count 22

would show you how many interfaces are assigned to Vlan 22, but not how many hosts are actually currently active in that subnet. I'll try and come up with a regular expression that gives you that output...

[Leo Laohoo]

1. "sh ip arp" on the router. 
2. Check the DHCP scope for active leases.
3. Scream test - Shut down the VLAN interface and wait for people to scream.

[asimGurung]

Hi,

You could try the show mac-address-table command. However, if you have multiple VLANs with inter-VLAN routing configured, it will not produce a reliable result.

Alternatively, try pinging 10.10.10.0 network from the switch, so that you can identify the hosts based on the replies.

Reply to request 4 from 10.10.10.1, 0 ms

Reply to request 4 from 10.10.10.2, 0 ms

Reply to request 4 from 10.10.10.3, 0 ms

Reply to request 4 from 10.10.10.5, 0 ms

[paul driver]

Hello
you should know how many hosts are allowed on a particular subnet through understanding CIDR notation -

To see what interfaces are assigned to a particular vlan on a switch you can just run

sh vlan brief

To see how many hosts are active for a particular vlan on the network you can run these commends on the device performing the inter-vlan routing 

sh ip arp <interface> x
sh ip arp | in Vlanx

[Leftz]

pinging 10.10.10.0 network from the switch is also a good idea. but when I do it, it does not work in all situations. For example, in this situation it cannot work. The connection is like this: SW1 ----- SW2 ----- APs. SW1 is layer 3 SW and SW2 is layer 2. APs gateway (10.10.1.1/24) is located at SW1. AP1 ip address is 10.10.1.10, AP2's is 10.10.1.11, and so on. SW1 can ping single ip address 10.10.1.10, but cannot get reply when ping 10.10.1.0. Any explanations? Someone mention there is broadcast ping, but I cannot remember it. 

Thanks

[Joseph W. Doherty]

With 100% accuracy (at least with hosts currently on-line)?

Not, I believe, with a single command.

You're best bet would be to use something that ARPs every host IP on the network of interest.  This might be accomplished, indirectly, by some ping utilities.  Although ping replies would show hosts, some hosts might chose not to reply to the ping scan.  However, all should respond to the ARP request.  After the "ARP scan" check the ARP table.