e Internet called the mul- ticast backbone (mbone). The mbone system conserves bandwidth over a distance, relieves congestion on transit links, and makes it possible to address a large population in a single multicast. Class E-Class E is address space reserved for experimental purposes. Class A 1-126 16,777,214 Class B 128.0-191.255 16,384 65,534 Class C 192.0-223.255.255 2,097,152 Class D Multicast 224-239 Class E Reserved 240-247 Figure 8.12 IPv4 32-bit addressing Chapter 8 The Internet and IP Infrastructures The digits in an IP address tell a number of things about the address. For exam- ple, consider the IP address 124.29.88.7. We know this is a Class A network because the first octet, 124, falls in the range of numbers for Class A addresses. Thus, 124 indicates the network ID. The remaining three octets, 29.88.7, indicate the host ID. In the address 130.29.88.7, the first two octets, 130.29, comprise the network ID and indicate that this is a Class B address; the second two octets, 88.7, comprise the host ID. Figure 8.13 shows an example of IP addressing on a network. Network IDs are managed and assigned by the RIRs (ARIN, APNIC, RIPE NCC, LACNIC, and AfriNIC). Host IDs are assigned locally by the network admin- istrator. Given a 32-bit address field, IPv4 makes available approximately 4.3 bil- lion different addresses. That seems like a lot, but as we began to experience growth in the Internet, we began to worry about the number of addresses left. In the early 1990s, the IETF began to consider the possibility of running out of IP address space. The result was the implementation of CIDR, which eliminated the old class-based style of addressing. A CIDR address is still a 32-bit IP address, but it is hierarchical rather than class based. Large national and regional service provid- ers are allocated large blocks of contiguous Internet addresses, which they then allocate to other smaller ISPs or directly to organizations. Networks can be broken down into subnetworks, and networks can be combined into supernetwor