X.121
（重定向自International Data Number）
X.121 is the ITU-T address format of the X.25 protocol suite used as part of call setup to establish a switched virtual circuit between Public Data Networks (PDNs), connecting two network user addresses (NUAs). It consists of a maximum of fourteen binary-coded decimal digits and is sent over the Packet Layer Protocol (PLP) after the packet type identifier (PTI).