For Macintoshes, there are now 2 or 3 vendors selling a SLIP module to work with Apple's MacTCP software. There are also other, and probably better, ways to get a SLIP service than is described here. Some terminal server hardware includes SLIP support.
Some commercial venders sell dialup IP service.
Your mileage will vary depending on the Unix workstation you wish to set up as SLIP server. The Mac side is much easier. Suppose your organization's class B network number is 139.123, and you are subnetting with a class C-sized network mask of 0xffffff00. A modem is attached to one of SunGod's serial ports. This rather important step is not discussed in the slipware documents.