tributed Data Interface (FDDI) within a campus or high-rise office building. The Metropolitan Area Network linking networks with either a leased line (such as T1, T3, or SONET) or Switched Multimegabit Data Services (SMDS). The WAN using Frame Relay, ISDN, or ATM and now GPRS and High-Speed Circuit-Switched Data (HSCSD). Regardless of the topologies, TCP/IP is robust enough to link all the var- ious systems together without any proprietary protocols. IP uses several different ways of handling the data transmission across a network. Unlike the OSI model, the X.25, the circuit-switched or leased-line networks where a connection is established between the two endpoints, IP uses a connec- tionless-oriented protocol. As a connectionless-oriented service, IP does not establish a true connection with the far end receiving the data. IP sends datagrams into the network with a best attempt to deliver the receiving end. IP does not know if the device exists or if it is online. IP makes no guarantees that the data will ever be delivered. IP does not con- cern itself with the integrity of the data. In X.25, every node processing packets checked the data integrity and the sequencing. IP does not. It will deliver bad or corrupted data to the far end, if the datagram is deliverable at all. IP does manage to route data through and to dissimilar networks. Whether the user is running a mainframe with SNA or a UNIX platform, IP does not get involved with the data differentiation. Another difference between IP and other protocols is that IP does not deal with the correct sequencing of the datagrams. Every datagram in the IP world is its own entity. Datagrams are not numbered 1, 2, 3, and SO on, as one will find in the X.25 packet-handling mechanisms and protocols. For all intents and pur- poses, IP is referred to as a dumb protocol because it does nothing except attempt to deliver the data. TCP sends its segments to IP. All it really tells IP about the data is the address. IP does not consider content. Its job is sim- ply to get the 