[DarkDeviL]

On the server that has both x.x.x.170 and x.x.x.177, you need to configure source based routing in the outound direction depending on the origin IP address, and assign that 172.16.20.3 to be routed out from x.x.x.177.

Whether it can be done with your setup depends on how it's done and what kind of systems and/or software/hardware firewalls you are using.

When the source based routing has been configured properly, Steam will see any connections from 172.16.20.3 to be done via x.x.x.177.

Your question is NOT related to Source Servers (SRCDS), or any other section on AlliedModders, as it involves network configurations and not the game server itself.

In order to figure out how, you are looking to find somewhere else, where they support your current setup (e.g. the "firewall" you mention), and ask them how to accomplish the source based routing.