Differentiating Between a Forward and Reverse Connection to a Modem

A host can make reverse-Telnet protocol connections to devices attached to a Cisco access server. Different port numbers (20xx, 40xx, and 60xx) are used for different device types. This is because each type has its own unique data type and protocol negotiations. The remote host must specify a particular TCP port on the router to connect with individual lines or a rotary group. For example, the remote host might make a reverse-Telnet connection to the modem using port 2097. The TCP port number 2097 specifies a Telnet connection (TCP port 2000) to line 97.

For the Telnet protocol, the base TCP port for individual lines is 2000, and the base TCP port for rotary groups is 3000. If the service provided is the raw TCP protocol (no Telnet), the base TCP port for individual lines is 4000, and the base TCP port for rotary groups is 5000. Telnet protocol (binary mode) uses 6000 as the base TCP port for individual lines and 7000 as the base TCP port for rotary groups. The Xremote protocol uses 9000 as the base TCP port for individual lines and 10000 as the base TCP port for rotary groups.

Router(config-line)#transport input {all | lat | mop | nasi | none | pad |
  rlogin | telnet | v120}

For example, if you enter the command transport input all, all possible command option protocols can be used for the connection. The command options are lat | mop | nasi | none | pad | rlogin | telnet | v120. Each command option protocol can also be specified individually.