interni

1. (of an organization) internal; in-house
   Ovaj dokument je samo za internu upotrebu. — This document is for internal use only.
   Pozovite me na interni broj 214. — Call me on extension 214.
2. (in medicine) internal medicine
   Ležao je nedelju dana na internom odeljenju. — He spent a week on the internal medicine ward.

For the physical inside of something, unutrašnji is usually used rather than interni.