The Point-to-Point Protocol (PPP) is a data link layer protocol that's designed to run on top of a simple serial link as physical layer.  Its main functionality is defined in [RFC 1661](https://datatracker.ietf.org/doc/html/rfc1661).

Since there are exactly two nodes on that point-to-point link, no addressing is required.

The PPP header does have an 8-bit address field but it's not used anywhere (always 0xff).