In the possessive case, IP is typically used as an informal abbreviated form of IP address. Example: His IP is 1.2.3.4.
This usage is typically found on IRC and similar real-time, textual chat media, where such abbreviations are used to save typing, and references to IP addresses are common. This usage could conceivably be used in vocal communication as well (e.g. on a telephone or face to face), but in this case, the means of communication does not typically involve IP addresses (with the notable exception of voice over IP), so the concept is not usually relevant.