If you did i.e. a "telnet" to another host, but want to do a "ls -l" on the current host, you do not have to close the "telnet" session.

Just type "^Z" (control-z) to suspend the current telnet session. The command is then waiting for you to resume. In this time you get a shell prompt and can issue any command.

If you are ready to continue, you can resume the "telnet" session with the command "fg %1" ("foreground: the first command waiting").

If you want to run the command in the background, you could issue a "bg %1" ("background: first command waiting"), too.

The command "jobs" lists all commands waiting.