For example, LAN users can be given Internet access, while blocking all other Incoming traffic. User-requested packets are accepted, and all others are dropped, including those sent by attackers. Packer filters can control specific network applications. For example, ftp is used to copy files between computers on a network. A packet filter can block ftp requests sent to computers that contain sensitive information, so ftp cannot he used to copy data from those computers.
Packet filters have a number of advantages.
They are transparent to users, and easy to manage in simple environments. Packet filtering routers can be inexpensive because they are often already in the network, and do not require the purchase of new hardware. If many access rules are required, the processing required to screen packets can significantly degrade router and network performance. Finally, most routers don't support logging or automated testing. Without these features, it is difficult to detect and correct router setup errors.