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.

The Internet Assigned Number Authority (IANA) hosts the registry for special numbers for many protocols and many Internet Engineering Task Force (IETF) standards.

• BGP: https://www.iana.org/protocols#:~:text=Border%20Gateway%20Protocol%20(BGP)%20Parameters
• DNS: https://www.iana.org/protocols#:~:text=Domain%20Name%20System%20(DNS)%20Parameters
• DNSSEC: https://www.iana.org/protocols#:~:text=Domain%20Name%20System%20Security%20(DNSSEC)%20Algorithm%20Numbers
• IP Addresses: https://www.iana.org/numbers
• IPv4 Address Space: https://www.iana.org/protocols#:~:text=Internet%20Protocol%20version%204%20(IPv4)%20Address%20Space
• IPv4 Protocol: https://www.iana.org/protocols#:~:text=Internet%20Protocol%20Version%204%20(IPv4)%20Parameters
• IPv6 Protocol: https://www.iana.org/protocols#:~:text=Internet%20Protocol%20Version%206%20(IPv6)%20Parameters
• MIME: https://www.iana.org/protocols#:~:text=Multipurpose%20Internet%20Mail%20Extensions%20(MIME)%20and%20Media%20Types
• TCP: https://www.iana.org/protocols#:~:text=Transmission%20Control%20Protocol%20(TCP)%20Parameters
• TLS: https://www.iana.org/protocols#:~:text=Transport%20Layer%20Security%20(TLS)

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.