The reason to an application gateway must be work along with a packet filter is to provide high level security.
• Packet filter can check only content of IP, TCP and UDP headers to apply filtering. That is packet filter can provide packet-level security.
• If an organization provides some privileges to certain set of internal users to connect an application outside the network, for example telnet service, the router have to check whether the user have permission or not, when a user tries to utilize this service.
• That is, before using the service, the user must be authenticated themselves. This authentication process can’t be done by packet filters, because the user identity details will be included in the application layer data, but not in IP/TCP/UDP header data.
• But, an application gateway can check application level data and makes policy decisions based on the application data.
• For different applications, different application gateways are required.
Therefore, to provide application level security, application gateways must be work with packet filters.
If you found this answer helpful, please upvote and share with other students in your network.