Host groups are identified by class D IP addresses, i.e., those with "1110" as their high-order four bits. Class E IP addresses, i.e., those with "1111" as their high-order four bits, are reserved for future addressing modes.
In Internet standard "dotted decimal" notation, host group addresses range from 184.108.40.206 to 220.127.116.11. The address 18.104.22.168 is guaranteed not to be assigned to any group, and 22.214.171.124 is assigned to the permanent group of all IP hosts (including gateways). This is used to address all multicast hosts on the directly connected network. There is no multicast address (or any other IP address) for all hosts on the total Internet. The addresses of other well-known, permanent groups are to be published in "Assigned Numbers".
Appendix II contains some background discussion of several issues related to host group addresses.