The provided endpoint URL (https://internal.network/...) indicates that the target API is hosted on a private, internal network. By default, Appian Cloud cannot access resources on a private network over the public internet. A Virtual Private Network (VPN) tunnel is the standard Appian-supported solution to establish a secure and private connection between an Appian Cloud instance and a customer's self-managed environment. This tunnel allows the Appian instance to securely communicate with the internal network as if it were a part of it, thus resolving the connectivity issue.

A. Changing the API protocol from REST to SOAP does not address the fundamental network connectivity issue of reaching a private endpoint from the cloud.

B. The requirement is to integrate with an existing application in the customer's environment, not to migrate or redeploy the service into Appian Cloud.

C. IP allow-listing is a security measure for publicly accessible endpoints. It cannot make a private, non-internet-facing endpoint reachable from Appian Cloud.

1. Appian Documentation

Appian Cloud VPN Integration: "Appian Cloud provides customers with the ability to establish a secure connection from their Appian Cloud instance(s) to their private network resources via an IPsec VPN tunnel. This allows Appian Cloud to function as a node on your private network

securely accessing your resources." (See the "Overview" section).

2. Appian Documentation

Integrating with other systems: This page outlines various integration methods. For on-premise systems without public endpoints

it implicitly points towards solutions that bridge the network gap

with VPN being the primary method for secure

direct access. (See sections on "Web Service Integration" and related security considerations).