PPP permits multiple network layer protocols to operate on the same communications link. For every network layer protocol used, PPP uses a separate NCP, as shown in Figure 1. For example, IPv4 uses the IP Control Protocol (IPCP) and IPv6 uses IPv6 Control Protocol (IPv6CP).
NCPs include functional fields containing standardized codes to indicate the network layer protocol that PPP encapsulates. Figure 2 lists the PPP protocol field numbers. 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.