 Protocol) A protocol that allows Internet hosts participate in multicasting. It describes the basics of multicasting IP traffic, including the format of multicast IP addresses, multicast Ethernet encapsulation, and the concept of a host group (i.e., a set of hosts interested in traffic for a particular multicast address). (interior gateway protocol) Any protocol used to propagate network reachability and routing information within an autonomous system. RIP and IGRP are examples of IGPs. IGRP (Interior Gateway Routing Protocol) A proprietary network protocol developed by Cisco Systems that is designed to work on autonomous systems. IGRP is a distance-vector routing protocol. IKE (Internet Key Exchange) The key exchange protocol used by IPsec. IKE supports pre- shared keys, which is a simplified form of key exchange. It does not require digital certificates. ILEC (incumbent local exchange carrier) A telephone company that was providing local service in the United States when the Telecommunications Act of 1996 was enacted. For most residents in the United States, this would be one of the four "baby Bells"-Qwest, SBC, BellSouth, and Verizon. IMA (Inverse Multiplexing over ATM) A specification that provides a way to combine ATM cell stream over two or more circuits, thus allowing an organization to lease just the band- width it needs (e.g., more than T-1 and less than T-3). immersion In virtual reality, the user's subjective sensation of being inside the virtual world and not observing it from an outside perspective. i-mode A proprietary protocol for transforming Internet information SO that it can be dis- played on the small screen of a mobile telephone or other portable device. i-mode is used in Japan and is also called DoCoMo (which means "anywhere"). impairment impairment A radio signal quality problem caused by degradations due to path loss, multi- path, adverse weather, obstacles, interference, or other obstacles. IMPS (Instant Messaging and Presence Service) A mobile phone standard that enables instant messagi