The FidoNet nodelist is distributed weekly to nodes of FidoNet, a network of BBSs in operation since the 1980s, giving information about all the nodes (BBSs) in the network. Its format is defined in FidoNet standards documents.
There was no single file extension for nodelists; rather, the filename format was NODELIST.nnn', where nnn was a three-digit number from 001 to 366 giving the number of the day of the year (starting with January 1) that the nodelist was released. Usually the nodelist was distributed in a compressed form (originally ARC and later ZIP), and in that case the filename replaces the first digit of the extension with "A" for ARC, Z for ZIP, or J, L, or R for ARJ, LZH, or RAR respectively if they are used (though they are not the standard compression methods for nodelist distribution).
The nodelist (once any compression is decompressed) is a plain ASCII text file with lines separated with CR (hex 0D) followed by LF (hex 0A), with the file terminated with Ctrl-Z (hex 1A). No other control characters or characters outside the 7 bit ASCII range are permitted. (Which means that even the current-day remnant of Fidonet which seems to be largely in Russia is saddled with a nodelist format that uses only Latin letters, not Cyrillic.)
Lines are limited to 157 characters (though it's recommended that nodelist processing programs support lines up to 1024 characters, and also tolerate files without the Ctrl-Z at the end).
Lines starting with a semicolon (;) are comments, and the next character(s) (if non-blank) signifies which audience the comment is intended for: S for sysops, U for users, F for comments intended to be included in formatted Fido lists, A for general interest, E for error messages. You can include multiple audience target letters (end with a blank).
Other lines are nodelist entries, consisting of these fields (comma-delimited):
1) Node type keyword
2) Node number
3) Node name
5) Sysop name
6) Phone number (starting with country code, formatted with dashes), or -Unpublished-
7) Modem speed (allows numbers from 300 to 33600 in a doubling sequence, but numbers over 9600 are rarely used because some nodelist processing programs are so ancient they are unaware that faster modems exist and pitch a fit on encountering them; anyway, the flags in the next field specify the particulars of modem capabilities)