We don't need MAC addresses in PPP since there are only two nodes, one at each end of the link. When using IP, do the nodes on a PPP link need IP addresses? Why or why not?
We do need IP addresses. The network layer at each end expects to see an IP header, which contains two IP addresses. A receiver would not know who the packet is intended for without a destination address, and, in the case of a router, could not forward it without a destination IP address. A receiver would not know who sent it without a source IP address.