When does packet filtering offer an advantage over other security methods, such as proxy services?

When does packet filtering offer an advantage over other security methods, such as proxy services?
Packet filtering can be configured with rules affecting all protocols in the TCP/IP, making them a good
first line of defense on the perimeter. The advantage of packet filtering is that it is low cost and it has
low impact on network performance. Proxy servers, which work on the application layer of the OSI
model, are much less flexible.

To transfer information across a network, the information has to be broken up into small pieces, each of which is sent separately. Breaking the information into pieces allows many systems to share the network, each sending pieces in turn. In IP networking, those small pieces of data are called packets. All data transfer across IP networks happens in the form of packets.
The basic device that interconnects IP networks is called a router. A router may be a dedicated piece of hardware that has no other purpose, or it may be a piece of software that runs on a general-purpose UNIX or PC (MS-DOS, Windows, Macintosh, or other) system. Packets traversing an internetwork (a network of networks) travel from router to router until they reach their destination. The Internet itself is sort of the granddaddy of internetworks – the ultimate “network of networks.”
A router has to make a routing decision about each packet it receives; it has to decide how to send that packet on towards its ultimate destination. In general, a packet carries no information to help the router in this decision, other than the IP address of the packet’s ultimate destination. The packet tells the router where it wants to go, but not how to get there. Routers communicate with each other using “routing protocols” such as the Routing Information Protocol (RIP) and Open Shortest Path First (OSPF) to build routing tables in memory to determine how to get the packets to their destinations. When routing a packet, a router compares the packet’s destination address to entries in the routing table and sends the packet onward as directed by the routing table. Often, there won’t be a specific route for a particular destination, and the router will use a “default route;” generally, such a route directs the packet towards smarter or better-connected routers. (The default routes at most sites point towards the Internet.)

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