A DNS record
type as per RFC 1035
ISDN in this context is short for Integrated Services Digital Network. As the name implies, this rare DNS record is used to keep track of ISDN nodes on a network. The value of a ISDN record is simply the node's complete telephone number (since ISDN uses the public telephone network). This is commonly used in conjunction with a RT record in order to make the record useful on networks such as the Internet.