I have some questions about the tool of Megablast in NGS:Mapping when I download the needed indexes .

There are four indexes files we need for Megablast:htgs 28-Jan-2013 ,nt 28-Jan-2013,wgs 28-Jan-2013,phiX174;but there are too many types of the indexes file in ftp://ftp.ncbi.nlm.nih.gov/blast/db/ ,for example : nt.00.tar.gz,nt.01.tar.gz,nt.02.tar.gz....I don't know the differents among these files ,and I don't which one  I should choose to download.In addition ,I can't find the index file for phiX174.

Could anyone help me and give me some advice please? Thank you very much.