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Classical IP over ATM, or CIP as it is informally called, defines how to transmit IP datagrams over ATM networks. CIP was originally defined in RFC 1577 (Classical IP and ARP over ATM, January 1994). This RFC was made obsolete and replaced by RFC 2225 (Classical IP and ARP over ATM, April 1998).

CIP is an IETF standard. Alternative methods for integrating ATM into legacy networks have been developed by the ATM Forum and are outlined under the heading "IP over ATM." An ATM Forum standard called LANE (LAN Emulation) provides a way to integrate ATM with legacy LAN protocols such as Ethernet and Token Ring (and carry IP as if the ATM network were a LAN). Another ATM Forum standard is MPOA (Multiprotocol over ATM), which adds cut-through/shortcut routing to the LANE scheme. Both are discussed under separate headings.

CIP uses an ATM network as its underlying data link network. In this scheme, ATM is like any other data link network such as Ethernet and token ring. CIP implements the concept of a LIS (logical IP subnet), which is a closed logical IP subnetwork (such as a department or workgroup) consisting of a group of hosts. Multiple LISs can exist on the same ATM network, but routers are still needed for inter-LIS communications.

There is some inefficiency in this configuration. If two LISs are on the same ATM network, a host on one LIS must go through a router to communicate with a host in the other LIS, even though the underlying ATM network is capable of setting up a virtual circuit that directly connects both hosts. This is by design, in order to retain the requirement that packets addressed to hosts in other subnets be sent to a default router. This is why it is called classical IP.

Later, the IETF defined NHRP (Next Hop Routing Protocol), which can set up direct ATM virtual circuit connections between end stations in different LISs. This is pictured in Figure 1.