The ISDN RRs require the following RDATA fields:

• <ISDN-address> (identifies the ISDN number of <owner> and DDI (Direct Dial In) if any, the format in master files is a <character-string> syntactically identical to that used in TXT and HINFO)
• <sa> (specifies the subaddress (SA), the format of <sa> in master files is a <character-string> syntactically identical to that used in TXT and HINFO)

The format of ISDN is class insensitive. ISDN RRs cause no additional section processing.

More information on the ISDN record is documented with RFC 1183.