MacIP encapsulates TCP/IP in AppleTalk packets for transmission over LocalTalk or similar cabling Open Transport can perform the MacIP packing/unpacking on your PowerBook and on your Performa 6115.

However, you'll need some extra software on the Performa 6115 to route the unpacked MacIP packets from the serial interface to the ethernet interface. IPNetRouter is a popular solution, but a bit expensive. All of the other software alternatives have been discontinued. Hardware routers (eg GatorBox or FastPath) are an even better solution but are hard to find nowadays.