Worm intrusion has become an increasingly severe threat to the Internet. The active worms propagate quickly, creating potential damage within minutes in a network. Network sniffers are programs that read packets traveling across a network and it analyze packets , which can capture, parse, and display the packet header information. A router-based worm detection system is proposed in which a network sniffer implemented in the LAN detects worm-probing traffic automatically by matching destination port numbers between incoming and outgoing connections.