How to Detect a Sniffer and How to block Sniffer?

There are a few ways a network technician can detect a NIC running in promiscuous mode. One way is to physically check all the local computers for any sniffer devices or programs. There are also software detection programs that can scan networks for devices that are running sniffer programs (for example, AntiSniff). These scanner programs use different aspects of the Domain Name Service and TCP/IP components of a network system to detect any malicious programs or devices that are capturing packets (running in promiscuous mode). However, for the average home user, there is really no way to detect whether a computer out on the Internet is sniffing your information. This is why encryption is strongly recommended. 

There is really only one way to protect your information from being sniffed: Use encryption! Using Secure Sockets Layer (SSL)-protected Web sites and other protection tools, you can encrypt your passwords, email messages and chat sessions. There are many programs available for free that are easy to use. Although you do not always need to protect the information passed during a chat session with your friends, you should at least have the option available when needed. 

Because of the very nature of a WLAN, encryption is a must in any situation. Fortunately, wireless networks come with the option of encryption built right into their software. However, few take advantage of this capability, as few are even aware that this option exists.