I couldn't say it shorter and more clearly than Vint : PPP does NOT belong to the TCP/IP protocol suite. It's a Layer 2(*) protocol, intended to carry multiple Layer 3 protocols (like IP, IPX,...) over a point-to-point connection (like PSTN, ISDN,...). PPP succeeded SLIP by bringing extended features : SLIP could only encapsulate IP while PPP can encapsulate several protocols, PPP supports authentication while SLIP didn't, etc. Remember that TCP/IP only covers Layer 3 up to Layer 7 : it's designed to be implemented on existing lower layers (1 and 2) : LAN (Ethernet, Token Ring, Wireless Lans,...) or WAN (ISDN, ATM, Frame Relay,...). E.T. (*) Those layers always refer to the OSI model. Think of a Layer 2 or 3 or 4 Switch : it doesn't depend on the protocol suite above, so we always refer to the vendor- technology- protocol-independent OSI reference model. --- The basic answer ends here --- Now a small yet technical recall : when data comes from an application to be transported on a physical medium (copper cable, fiber optics, radio waves, infra-red,...), on its way from Layer 7 to Layer 1 it reaches IP (Layer 3) that encapsulates it in a datagram/packet and specifies the destination network+host address. Then it's forwarded to PPP (Layer 2) that encapsulates it in a frame and specifies the way bits are organized to travel through the physical medium. Then it's forwarded to some Layer 1 technology that converts the bits into a specific signal using a specific encoding scheme (V.90 on PSTN, I.430 on ISDN BRI,...) and finally reaches the physical medium to be physically transported through the network. --- The extended answer ends here --- -----Original Message----- From: vint cerf [mailto:vinton.g.cerf@wcom.com] IP is encapsulated in PPP for all practical purposes. PPP can support multiple protocols on a single point to point link in the same way ethernet can support multiple protocols vint At 08:01 AM 3/1/2002 -0500, Bill Cunningham wrote: >Is IP actually encapsulated in PPP, or is PPP and IP sent out at the same >time at different protocol layers? Kinda holding hands in a sense to each >other.