This analogy explains the two main functions of a proxy server. First, the proxy server acts as an intermediary, helping users on a private network get information from the Internet when they need it, while ensuring that network security is maintained. Second, a proxy server may store frequently requested information in a local disk cache, rapidly delivering it to multiple users without having to go back to the Internet to get it. Because proxy servers and firewalls are so often bundled together, people often confuse the two. However, a packet-filtering firewall operates at the Network layer of the OSI model, while proxy servers work at the Application layer.
Packet filters use routers to filter information coming to and from a network.
On the other hand, creating complex access rules can be difficult. Further, all packet filters can do is grant or deny access based on a packet's apparent source or destination address. Hackers can fool such firewalls by forging source addresses via IP packet spoofing. Since client-server connections are direct, hackers can also use packet sniffers to discern a network's address structure with relative ease. In our library analogy, the equivalent of a packet-filtering firewall would be the librarian keeping a list of trusted students, then allowing only those individuals into the locked stacks to retrieve books.