Show history of IP addresses assigned to iMac for a period of time? Is there a way to view the list of IP addresses that were assigned to my iMac over a period of time (e.g., the last 30 days)? I'd like to get this information from the iMac itself if possible.
By default your mac won't keep a history of DHCP address and times.  I put together a launchd and script that should do the trick if you must have client side records for DHCP times and ips.

launcd plist called com.local.DHCP-History.plist that needs to be placed in /Library/LaunchDaemons/

&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"&gt;
&lt;plist version="1.0"&gt;
&lt;dict&gt;
    &lt;key&gt;Disabled&lt;/key&gt;
    &lt;false/&gt;
    &lt;key&gt;Label&lt;/key&gt;
    &lt;string&gt;DHCP_History&lt;/string&gt;
    &lt;key&gt;Program&lt;/key&gt;
    &lt;string&gt;/usr/bin/DHCPhistoryLog.sh&lt;/string&gt;
    &lt;key&gt;WatchPaths&lt;/key&gt;
    &lt;array&gt;
        &lt;string&gt;/private/var/db/dhcpclient/leases/*&lt;/string&gt;
    &lt;/array&gt;
    &lt;key&gt;WorkingDirectory&lt;/key&gt;
    &lt;string&gt;/usr/bin/&lt;/string&gt;
&lt;/dict&gt;
&lt;/plist&gt;


Here is the script it will run, I put it in /usr/bin/ but you can put where you want just make sure to change the environment path in the launch daemon.  You will have to create a folder called DHCP_History in /private/var/log/ to create the log files in.

DHCP_History.sh

#!/bin/bash
#
# tron_jones 09-10-14

# Create a log for DHCP changes and put in new folder with date
dateVar=`date` 
mkdir /private/var/log/DHCP_History/"$dateVar" 
path="/private/var/db/dhcpclient/leases/"
logPath="/private/var/log/DHCP_History/"
for i in `ls /private/var/db/dhcpclient/leases/`
do
    cp "${path}${i}" "${logPath}${dateVar}"/DHCP_ChangeHistory_"${i}".log
done


Make sure that the script is executable and owned by root:wheel.  Run these commands to do that:

sudo chmod +x /usr/bin/DHCP_History.sh

sudo chown root:wheel /usr/bin/DHCP_History.sh


Now every time DHCP gets renewed or changes it will create a file called DHCP_ChangeHistory.log inside /private/var/log/DHCP_History/thedate/