Security is one of the key features that has to be considered during the Network design. If you have servers in your LAN which are accessible from outside , you should certainly consider deploying some sort of security . This could range from applying access-lists on your perimeter router to having a PIX firewall or using the Intrusion detection systems.
You could use a sniffer to sniff packets if you want to check the packets coming into the LAN. Free downloads are available. But I would recommend that you consider implementing some sort of Security into the system.
