You have two options.
You can NOT use PPP, but rather connect
via an async tty to the 2511 port and then
autocommand to the Win2k server (the autocommand
can specify a port, stream, quiet, etc).
Or, you can have a PPP connection between the
2511 and W98, and IP traffic can pass from the
Win98 to the Win2k server (this is not a PPP end to
end connection). Each device will have it's
own IP address, the 2511 will simply route the
packets.