Here’s a quick tip to help you identify an open IP address on your network!
*** Disclaimer *** You should always check with your network team first to see if they already have a list of IP’s to use.
First, figure out what subnet and subnet mask your IP will come from. If you’re not sure what the IP ranges are check out this subnet calculator.
Second, ping some IP’s that you think would be suitable. Once you find one that does not respond to a ping move on to the next step.
Third, open a command prompt and run an ‘nslookup‘. We run an nslookup in order to determine if there is already a DNS entry associated to that IP. In some cases servers or workstations are turned off at the time you ping them so don’t skip this step.
Below is an example, bold text is what you enter. In the example I’m looking up the IP address 192.168.1.100 to see if there are any DNS records and the result shows that there are no DNS entries for that IP address. If this is the result you get from your own nslookup then you can be fairly confident that you can use the IP that you just looked up.
Example 1: The IP does not have a DNS record
C:\> nslookup
Default Server: mydomain.com
Address: 192.168.1.1
> set q=any
> 192.168.1.100
Server: mydomain.com
Address: 192.168.1.1
*** mydomain.com can’t find 192.168.1.100: Non-existent domain
>
Here is an example of an IP that does have a DNS record. If you see this then you will have to go back to the second step and ping for another IP.
Example 2: The IP has a DNS record
C:\> nslookup
Default Server: mydomain.com
Address: 192.168.1.1
> set q=any
> 192.168.1.200
Server: mydomain.com
Address: 192.168.1.1
Name: server-a.mydomain.com
Address: 192.168.1.200
>