Errors while connecting R/3 with RFC

Dear All,

when i am tring to connect SAP R/3 tru RFC it is throwing a following error" Unable to establish connection to R/3 system 172.20.6.100 (system=00, client=500) Troubleshooting tips:

Is the target system online?

Check the connection data (note that server names and groups are case-sensitive)

Tips for administrators (see the configuration guide for more details):

Does the user have the required authorizations in the target system?

Is the target system configured correctly in "etc/services"? "

Plz tell me what might be the error & how to solve this?

Thanks,

nishu