If you are staying less than 4 months but need to arrange any affairs with Dutch government agencies, you will need a Citizen Service Number (BSN). This can be obtained by registering as a non-citizen (RNI).
If you are visiting the Netherlands from abroad to work or study and/or you will be staying less than 4 months, you are not required to register as a resident. However, in the case that you must arrange any affairs with Dutch government agencies, you will still need a Citizen Service Number (Burgerservicenummer - BSN). This can be obtained by registering as a non-citizen (Register Niet- Ingezetenen - RNI). You do not need a Dutch address for RNI registration.
You should be aware that an RNI registration means you will have a BSN but you will not appear in the Personal Record Database (Basisregistratie Personen - BRP). This can make getting health insurance in the Netherlands very difficult. However, health insurance is only mandatory after 4 months of living in the country.
You can contact the Municipality of Leiden, or one of 17 other selected municipalities to make an appointment to do this. These are: Alkmaar, Almelo, Amsterdam, Breda, Doetinchem, Eindhoven, Groningen, Goes, Heerlen, Leeuwarden, Nijmegen, Rotterdam, Terneuzen, The Hague, Utrecht, Venlo and Zwolle.
Registration as non-citizen is not offered at the ECL; you will need to call the Municipality of Leiden at +31 71 516 516 5, or one of the other municipalities mentioned above.
You will need to visit in person and present a valid proof of identity (a passport or identity card) and will be asked to provide your address outside of the Netherlands.