IPCP

IPCP, or Internet Protocol Control Protocol, is one of the Network Control Protocols used by the Point-to-Point Protocol (PPP) to configure and enable IPv4 over a PPP link. IPCP negotiates the configuration of the IP layer between the two endpoints, including the assignment of an IP addresses and optional DNS server information during the PPP establishment phase.

During PPP operation, after the Link Control Protocol (LCP) has established a link, the IPCP phase begins. The peers exchange Configure-Request messages containing IPCP options such as the local IP address, the remote IP address, and DNS addresses. The peer may respond with Configure-Ack to accept, or Configure-Nak/Reject to propose alternatives or reject options. This negotiation allows dynamic or static IP assignment depending on the arrangement between the communicating ends.

IPCP is IPv4-specific; IPv6 uses a separate protocol, IPv6CP, within PPP. IPCP does not provide authentication or encryption by itself; PPP authentication (such as PAP or CHAP) is handled by LCP or by higher-layer mechanisms. IPCP focuses on IP address configuration and related parameters; optional parameters include primary and secondary DNS addresses, domain search lists, and other IP-related configuration if supported by implementation.

In practice, IPCP is commonly used in dial-up connections, as well as some DSL or VPN contexts where PPP is employed. While many modern networks have migrated to other negotiation mechanisms, IPCP remains a foundational element of PPP for IPv4 connectivity and interoperability with legacy devices.