Internet Assigned Numbers Authority (IANA) tracks the assignment of all DNS related constants. This includes available classes, resource records types, OpCodes, RCODEs, and various other values. Each assigned number is linked to the corresponding RFC describing its function. This is a great overview to see which values are available.

Internet Assigned Numbers Authority (IANA) tracks the assignment of all NetFlow/IPFIX related constants. The website lists all available fields and describes their meaning. Each assigned number is linked to the corresponding RFC describing its function. This is a great overview to see which values are available.