An Internet Protocol address is assigned to every computer connected to the Internet, but these are frequently temporary and will change over time or if the computer is moved to a different location. Past IP addresses are not centrally recorded, but it is possible to determine a past IP address, along with the date and time it was assigned, whenever the computer in question has been used to send an email message.
Open an email message known to be sent from the computer or person whose past IP address you wish to determine.
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Change your settings for that message in your mail software to display the complete email headers for that message. An email header is attached to every Internet email message, and records the route the message took from the sender to the recipient. The command to show complete headers will vary depending upon the software you are using; look for menu commands such as "show full headers," "show Internet headers" or "show raw source."
Review the headers to find a series of entries that start with the word "Received." These document the trail the message took.
Read the first Received line for an IP address. This will be the IP address of the computer which originated the email message, at the time stated in the "Date" header.