The active file lists all groups available on the server in question. It also lists the highest and lowest current article numbers in each group.
Here’s an excerpt from a typical active file:
soc.motss 296030 293865 y alt.binaries.pictures.fractals 3922 3913 n comp.sources.unix 1605 1593 m comp.binaries.ibm.pc 5097 5089 y no.general 1000 900 y
Here’s a pseudo-BNF definition of this file:
active = *group-line group-line = group spc high-number spc low-number spc flag <NEWLINE> group = <non-white-space string> spc = " " high-number = <non-negative integer> low-number = <positive integer> flag = "y" / "n" / "m" / "j" / "x" / "=" group
For a full description of this file, see the manual pages for ‘innd’, in particular ‘active(5)’.