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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 4 (IP, IPv4)                      9  IP Addressing                           9  IP "Classful" (Conventional) Addressing

IP Reserved, Loopback and Private Addresses 1 2 Problems With "Classful" IP Addressing

The vast majority of traffic on IP internetworks is of the unicast variety: one source device sending to one destination device. IP also supports multicasting, where a source device can send to a group of devices. Multicasting is not used a great deal on the Internet as a whole at the present time, mainly due to lack of widespread hardware support, so most of our focus in looking at IP is on unicast. Multicast is useful in certain circumstances, however, especially as a more efficient alternative to broadcasting. I include one summary topic on multicasting for your perusal, and also want to briefly discuss here IP addressing issues related to multicasting.

The “classful” IP addressing scheme sets aside a full one-sixteenth of the address space for multicast addresses: Class D. Multicast addresses are identified by the pattern “1110” in the first four bits, which corresponds to a first octet of 224 to 239. So, the full range of multicast addresses is from 224.0.0.0 to 239.255.255.255. Since multicast addresses represent a group of IP devices (sometimes called a host group) they can only be used as the destination of a datagram; never the source.

The 28 bits after the leading “1110” in the IP address define the multicast group address. The size of the Class D multicast address space is therefore 2²⁸ or 268,435,456 multicast groups. There is no substructure that defines the use of these 28 bits; there is no specific concept of a network ID and host ID as in classes A, B and C. However, certain portions of the address space are set aside for specific uses. Table 48 and Figure 63 show the general allocation of the Class D address space.

The bulk of the address space is in the middle multicast range, which are “normal” multicast addresses. They are analogous to the Class A, B and C unicast addresses and can be assigned to various groups.

The last address range is for administratively-scoped multicast groups. This is a fancy term for multicast groups used within a private organization; this block, representing 1/16th of the total multicast address space, is comparable to the private addresses we saw in the preceding topic. This block is also subdivided further into site-local multicast addresses, organization-local addresses and so forth.

IP Reserved, Loopback and Private Addresses 1 2 Problems With "Classful" IP Addressing