add a route to a specific IP on linux to get to a remote emulated system

Note on how to add a route on a system remote to an host for an emulated system.

Use case.  A system with TCP/IP configured is running on a host with a local IP for the emulated system, and a gateway IP on the host.  The latter  is on a tun/tap NIC on the host system.

one can access the emulated system from the network on the host system.  But the gateway tun/tap device isn't shared on the external network.

example

[hercules guest]

ip xxx.201  NIC internal to OS

[CTCA bridge]

ip xxx.151  NIC tun/tap on host running hercules

[Host system]

ip xxx.136 NIC is enp0 ethernet NIC for host.

[subnet]

xxx/24

[remote system]

ip xxx.172

traffic for the xxx.151 ip isn't bridged out of the [Host system]

ip route add xxx.151 via xxx.136 dev enp0s25 executed on the [remote system]

allows programs on remote system access the [hercules guest]

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