It's contained in an HTTP header. However, IP addresses are not suitable to uniquely identify a user - aside from the spoofing issue, anyone managed by DHCP and/or NAT won't have a unique and unvarying IP address.
"privilege" comes from the Latin words for "private" and "law" (legal) and dates to feudal times. To "claim privilege" meant that you were above the laws that applied to the common people.
Put the moon back where you found it! We need it for tides and poetry and stuff. Like this tiny ad:
Devious Experiments for a Truly Passive Greenhouse!