Hai,
I want to setup a dial-in server & dial-up in same machine. As part of that I have set-up a dial-in server.The client can access my shell now.
The problems are,
1)But how can server monitor the where abouts of client?
2) But I'm not able to setup dial-up & dial-in server concurrently
by using one modem.

Seeking the co-operation of all,
jus.

It is possible, but you'll need to do some manual scripting..

If dial-in is running, pppd will be listening on the modem port ttyS~ and you won't be able to re-use that connection.
The trick is to kill that pppd process when you start the dial-up, and then restart it once you have finished...

A lot depends on your distribution and which files are controlling the ppp0 interface, but the theory is to find the ppd process, (quite easy if there is only one, eg pppd or pppd-watch, and much harder if you have several eg VPNs) and add a kill line in the startup script for pppd dial-up start and a start command in "pppd stop"...

Usually pppd dial-up will "call" a script in /etc/ppp/peers if it is listed in the config file...