IP Serial Library, included in the Nport Administrator.
For TCP mode, the IP Serial library is not suitable.
There is also no directly available library for that for Nport 5000 series.
However, and this is good news, there is an SSDK library for Nport 6000 series. S stands for secure, hence only available for Nport 6000, but it can be used for 5000 series too, if the encryption is not required. It can be used on linux and windows.
It can be downloaded from the Nport 6000 series web site: