To refer to the IPIP in general, one or more of the following should be sufficient for most purposes:
Goldberg, L. R. (1999). A broad-bandwidth, public domain, personality inventory measuring the lower-level facets of several five-factor models. In I. Mervielde, I. Deary, F. De Fruyt, & F. Ostendorf (Eds.), Personality Psychology in Europe, Vol. 7 (pp. 7-28). Tilburg, The Netherlands: Tilburg University Press.
Goldberg, L. R., Johnson, J. A., Eber, H. W., Hogan, R., Ashton, M. C., Cloninger, C. R., & Gough, H. C. (2006). The International Personality Item Pool and the future of public-domain personality measures. Journal of Research in Personality, 40, 84-96.
International Personality Item Pool: A Scientific Collaboratory for the Development of Advanced Measures of Personality Traits and Other Individual Differences (http://ipip.ori.org/). Internet Web Site.
Only a few scales on the IPIP site have been explicitly named by the scale author. For example, Johnson (2014) refers to his 120-item representation of the NEO Personality Inventory as the "IPIP-NEO-120." If a scale author has explicitly named an IPIP scale, one should use that name and cite the author.
Most scales, however, have not been explicitly named. In these cases, the best practice would be to refer to them by the scales on which they are based. For example, the sample questionnaire based on Goldberg's lexical factor markers could be referred to as "the 50-item IPIP representation of the Goldberg (1992) markers for the Big-Five factor structure" or something like that. In this example you would include the citation for the scale on which the IPIP measure was based (Goldberg, 1992) and also the Web page containing the table or key for the IPIP scale, using a standard format for citing a Web page.
One should never refer to a specific IPIP scale as "the IPIP." Please keep in mind that the IPIP stands for the International Personality Item Pool, which is a set of over 3,000 items.
One should never refer to a specific IPIP measure with an overly general label such as "the Big-Five IPIP Inventory." There are over a half-dozen IPIP inventories for measuring the Big-Five personality factors, so one should use a label that refers to the specific measure used in one's study.