even i m new to the world of microcontrollers. i m making a project with at89c51. but after learning about programming the uc i think its better to change to at89s52. even one professoe suggested the same to me. will it be ok to do it?

Yes, It will be much better since 'S52 supports ISP and has all the features of 'C52 plus some more.

are these 2 chips compatible with each other (i have completed hardware part n done some testing of software with at89c51)?

Compatibility in what sense?
Code compatible No?
Pin Compatible? Somewhat beware of the SPI in 'S52.


also, i want to make n hardware modules and put them on lan?
Did you mean LAN ethernet? Can be done with 8052 famility micros there are several open source projects on web do a search in google.

for this purpose which uc is best?
That depends on your requirement. Do a parametric search in KEIL device database for your requirements. Here is the Device database. http://www.keil.com/dd/

what actually is can?
CAN stands for controller area network. It is a communication standard like RS232, RS422 etc. It is widely used in automotive industry.

Regards,
Prahlad Purohit