The performance limits of the current Internet make the integration of IP with ATM a hotly debated issue in the networking arena, leading to various competing approaches and products. Legitimate technical and market issues are, however, often intertwined with biased views and hype, with vendors competing in the standards arena as well as on the markets. Together with the speed of technical evolution, this causes confusion for purchasers of networking equipment who, usually preferring a single vendor for their networks, run the risk of remaining locked into solutions that will not scale with the evolving needs and that will not fully inter-operate with other networks, even using the IP protocol. It is important to note that the term 'Internet' refers indeed to a specific network. In the following, we will refer more generally to the TCP/IP family of protocols, in order to encompass all types of IP networks such as intranets, extranets and the Internet. We will cover both IP version 4 (IPv4) and IP version 6 (IPv6). Unless Ipv6 is explicitly stated, IPv4 should be assumed. IP and ATM integration refers here to the support of IP over (or within) ATM. From our point of view, integration is a particular case of coexistence. Another particular case is IP and ATM interworking, which means the interoperation between applications or between complete protocol stacks, based on one side on the IP technology, and on the other side on the ATM technology. For instance, interworking between IETF IP based videoconferencing and ATM based videoconferencing. In this paper, we are not dealing with any interworking scenario. Interworking scenarios do not happen very often and are very difficult to realize. In fact, additional non-trivial aspects like interworking at the user plane, mapping of addresses, and several other issues strictly dependent upon the involved technologies, have to be considered in this case. The scope of this paper will thus be restricted to IP and ATM integration, and three levels of integration have been identified: * Level 1: use of IP over an intermediate layer over ATM (e.g. IP over LAN Emulation over ATM), * Level 2: use of IP directly over ATM (e.g. Classical IP, MARS, NHRP and MPOA), * Level 3: IP merged with ATM (e.g. MPLS). In the integration scenarios considered in this paper, the applications always use a common layer to communicate and, in the specific cases under investigation, this common layer is the IP layer. Therefore we use the term 'applications' to refer to IP applications. The present paper is derived from a so-called 'Guideline', NIG-3, produced by participants of the ACTS 'Chain' NIG: Global Network Interoperability (particularly projects CONVAIR, DIANA, EXPERT, IthACI, MULTICUBE and PETERPAN). ACTS (Advanced Communications Technologies and Services) represents the major telecommunications R&D focus of the European Union's 4th research framework Programme. ACTS uses 'Chains' of related projects as a primary vehicle for concertation, both within ACTS itself, and from ACTS towards groups investigating issues of major importance addressed by the communications sector at large. The Network Inter-operability (NI) Chain Group considers all issues of inter-operability between different types of networks. The current paper focuses on technology, and gives neither commercial, nor time-dependent information. It does not necessarily represent the views of the whole ACTS community or of any single organization participating in the Programme.