PPP Architecture - Network Control Protocol Layer
Point-to-point links tend to worsen many problems with the current family of network protocols. For instance, assignment and management of IP addresses, which is a problem even in LAN environments, is especially difficult over circuit-switched point-to-point links (such as dialup modem servers). PPP addresses these issues using NCPs.

PPP permits multiple network layer protocols to operate on the same communications link. For every network layer protocol used, PPP uses a separate NCP. For example, IP uses the IP Control Protocol (IPCP), and IPX uses the Novell IPX Control Protocol (IPXCP).
NCPs include functional fields containing standardized codes (PPP protocol field numbers shown in the figure) to indicate the network layer protocol that PPP encapsulates. Each NCP manages the specific needs required by its respective network layer protocols. The various NCP components encapsulate and negotiate options for multiple network layer protocols. Using NCPs to configure the various network layer protocols is explained and practiced later in this chapter.