Modern "Internet" is about about making everything tcp/ip based. Interconnection between networks with different protocols was a real thing long before tcp/ip conquered our planet:
"An amusing side note on the VNIIPAS connection: while the author of this paper was in Havana, he
connected to a VAXNMS system at his home via the following path: PAD program on Unix microcomputer
at CENIAI in Havana goes over X.25 board local to that system; X.25 line from Havana to Moscow, via
satellite; VNIIPAS X.25 data switch receives call, routes to international Sprint network via Western
Europe; Sprint carries call through some number of cities and links to Reston, Virginia where it conveys
call to Columbus, Ohio, to CompuServe's X.25 gateway; CompuServe carries call from Columbus, Ohio
to Tucson, Arizona, where it gets translated from X.25 formats to internal DECnet format and passes over
the University of Arizona DEC net network, through Ethernet, fiber optic, 56 Kbps synch and asynch
19.2 Kbps TCP/IP lines to author's home over another Ethernet from gateway to workstation, returning
with the prompt "Username:" The miraculous thing about this call is that it was done with a single X.121
address at the Havana end." - this is how it was in 80s.
"An amusing side note on the VNIIPAS connection: while the author of this paper was in Havana, he connected to a VAXNMS system at his home via the following path: PAD program on Unix microcomputer at CENIAI in Havana goes over X.25 board local to that system; X.25 line from Havana to Moscow, via satellite; VNIIPAS X.25 data switch receives call, routes to international Sprint network via Western Europe; Sprint carries call through some number of cities and links to Reston, Virginia where it conveys call to Columbus, Ohio, to CompuServe's X.25 gateway; CompuServe carries call from Columbus, Ohio to Tucson, Arizona, where it gets translated from X.25 formats to internal DECnet format and passes over the University of Arizona DEC net network, through Ethernet, fiber optic, 56 Kbps synch and asynch 19.2 Kbps TCP/IP lines to author's home over another Ethernet from gateway to workstation, returning with the prompt "Username:" The miraculous thing about this call is that it was done with a single X.121 address at the Havana end." - this is how it was in 80s.