Ibid

http://en.wikipedia.org/wiki/Ibid.

Ibid (Latin, short for ibidem, "the same place") is the term used to provide an endnote or footnote citation or reference for a source that was cited in the preceding endnote or footnote.

Example

4. E. Vijh, Latin for dummies (New York: Academic, 1997), p.23.
5. Ibid.
6. Ibid., p.29.

The reference in no. 5 is the same as in no. 4 (E. Vijh, Latin for dummies on page 23), whereas the reference in no. 6 refers to the same work but at a different location, namely page 29.