Packet Filtering

Packet filters either allow or block data packets often while routing them from one n network segment to another and most often between a private network and the Internet. Packet filtering can be done in a router or on an individual host computer using special software both implementations are considered a form of firewall, a secure barrier behind which our local area network operates in safety.

Packets can be filtered on the basis of :

• 
  
• Packet source address
• Packet destination address
• Source port number
• Destination port number

Packets can also be screened based on whether they're trying to initiate a connection. Before a packet can be screened, we must establish a set of rules that the router uses in blocking or allowing packets. These rules are usually stored in the router in a specific order and then applied in that same order once a packet is received so be sure the order makes sense.
Router vendors such as Cisco Wellfleet, 3COM, Digital Equipment Corporation (DEC) and many others supply routers that we can program to perform packet filtering functions.
Provide virus protection for all users, and scan all file servers and workstations daily. Use virus scanners that stay loaded and run all the time. Ensure all operating system patches are installed immediately when they're distributed. Don't expect the manufacturer to track we down and tell us about them.
Limit the services offered on our network to those that are necessary. Limit the number of logon account on the host. It is to be sure to look for and then remove any inactive accounts that we find.
All these recommendations have their place in our security plan but the best security plan in the world is worthless unless you carefully and precisely put it in place and then follow up by taking the appropriate steps to keep it working.