Internet Group Management Protocol(IGMP) An extension to the {Internet Protocol}, used by IP {hosts} to report their {host group} memberships to immediately-neighbouring {multicast} {routers}. See also {MBONE}. Version 1 of IGMP is defined in Appendix 1 of {RFC 1112}. Version 2 is proposed in {RFC 2236}. (1999-11-08)