Hi all , actually i am new to security , currently working in network field , learning about firewalling and all that stuff , can one please guide me that how to start in this.....thank you...!!!!
You could buy a book about security-basics and study that. The books that prepare for the Security+ exam are quite broad and are a good beginning for network-security also if you don't plan to take the exam.
Don't stop after you've improved your network! Improve the world by lending money to the working poor:
Once you've those study materials either via online or hardcopy, you might wanna install GNS3 on your latop. Please refer to http://www.youtube.com/watch?v=Eg46wzXyouw