IPv4 addresses have five classes: A, B, C, D, and E. In classful addressing, the most significant bits of the first byte determine the address class of the IP address. Table 1-9 shows the high-order bits of each IP address class.
Table 1-9 High-Order Bits of IPv4 Address Classes
| Address Class | High-Order Bits |
| A | 0xxxxxxx |
| B | 10xxxxxx |
| C | 110xxxxx |
| D | 1110xxxx |
| E | 1111xxxx |
Again, the IPv4 Class A, B, and C addresses are unicast addresses. Such an address represents a single destination. Class D is for multicast addresses. Packets sent to a multicast address are sent to a group of hosts. Class E addresses are reserved for experimental use. IANA allocates the IPv4 address space. IANA delegates regional assignments to the five Regional Internet Registries (RIR):
- ARIN (American Registry for Internet Numbers): Covers USA, Canada, and some Caribbean Islands
- RIPE NCC (Réseaux IP Européens Network Control Center): Covers Europe, the Middle East, and Central Asia
- APNIC (Asia Pacific Network Information Center): Covers the Asia Pacific Region
- LACNIC (Latin America and Caribbean Network Information Center): Covers Latin America and some Caribbean Islands
- AfriNIC (African Network Information Centre): Covers Africa
Updates to the IPv4 address space can be found at https://www.iana.org/assignments/ipv4-address-space/ipv4-address-space.xhtml.
The following sections discuss each of these classes in detail.
Class A Addresses
Class A addresses range from 0 (00000000) to 127 (01111111) in the first byte. Network numbers available for assignment to organizations are from 1.0.0.0 to 126.0.0.0. Networks 0 and 127 are reserved. For example, 127.0.0.1 is reserved for the local host or host loopback. A packet sent to a local host address is sent to the local machine.
By default, for Class A addresses, the first byte is the network number, and the three remaining bytes are the host number. The format is N.H.H.H, where N is the network part and H is the host part. With 24 bits available, there are 224 − 2 = 16,777,214 IP addresses for host assignment per Class A network. We subtract 2 for the network number (all 0s) and broadcast address (all 1s). A network with this many hosts will surely not work with so many hosts attempting to broadcast on the network. As discussed later in this chapter, subnetting can be used to define smaller networks within a larger network address.
Class B Addresses
Class B addresses range from 128 (10000000) to 191 (10111111) in the first byte. Network numbers assigned to companies or other organizations are from 128.0.0.0 to 191.255.0.0. This section discusses the 16 networks reserved for private use later.
By default, for Class B addresses, the first 2 bytes are the network number, and the remaining 2 bytes are the host number. The format is N.N.H.H. With 16 bits available, there are 216 − 2 = 65,534 IP addresses for host assignment per Class B network. As with Class A addresses, having a segment with more than 65,000 hosts broadcasting will surely not work; you resolve this issue with subnetting.
Class C Addresses
Class C addresses range from 192 (11000000) to 223 (11011111) in the first byte. Network numbers assigned to companies are from 192.0.0.0 to 223.255.255.0. The format is N.N.N.H. With 8 bits available, there are 28 − 2 = 254 IP addresses for host assignment per Class C network. H = 0 is the network number; H = 255 is the broadcast address.
Class D Addresses
Class D addresses range from 224 (11100000) to 239 (11101111) in the first byte. Network numbers assigned to multicast groups range from 224.0.0.1 to 239.255.255.255. These addresses do not have a host or network part. Some multicast addresses are already assigned; for example, routers running EIGRP use 224.0.0.10. You can find a full list of assigned multicast addresses at www.iana.org/assignments/multicast-addresses.
Class E Addresses
Class E addresses range from 240 (11110000) to 254 (11111110) in the first byte. These addresses are reserved for experimental networks. Network 255 is reserved for the broadcast address, such as 255.255.255.255. Table 1-10 summarizes the IPv4 address classes. Again, each address class can be uniquely identified in binary by the high-order bits.
Table 1-10 IPv4 Address Classes
| Address Class | High-Order Bits | Network Numbers |
| A | 0xxxxxxx | 1.0.0.0 to 126.0.0.0* |
| B | 10xxxxxx | 128.0.0.0 to 191.255.0.0 |
| C | 110xxxxx | 192.0.0.0 to 223.255.255.0 |
| D | 1110xxxx | 224.0.0.1 to 239.255.255.255 |
| E | 1111xxxx | 240.0.0.0 to 254.255.255.255 |
*Networks 0.0.0.0 and 127.0.0.0 are reserved as special-use addresses.
