The packet filter and firewall also ensure that packets that cannot be assigned to a local application are rejected. The public router IP, which also appears as the sender of data packets, is thus externally visible and addressable. Since unwanted network connections between local computers and the Internet are to be avoided as much as possible, the router IP usually comes between local IPs and destination addresses. The target application learns the sender port when the connection is established and sends the desired data packets back to the sender address. Open ports of target addresses “listen” for requests that originate via a port from private or public IPs. The destination address can be an app, a service, a website, or any other program such as a web browser. Transport protocols such as UDP (User Datagram Protocol) and TCP (Transmission Control Protocol) transfer data packets by assigning them to a specific address consisting of an IP and port number.
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