Information Technology Reference
In-Depth Information
Updates to the IPv4 address space can be found at www.iana.org/assignments/
ipv4-address-space.
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 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 re-
maining 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 2 24 - 2 = 16,777,214 IP addresses
for ho st as s ig nment per Clas s A net work. We s ubt rac t t wo for the net work 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. This section discusses subnetting
later as a method of defining 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 two bytes are the network number, and the re-
maining two bytes are the host number. The format is N.N.H.H. With 16 bits available,
there are 2 16 - 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. Net-
work 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 2 8 - 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. Net-
work 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, 224.0.0.10 is used by routers running EIGRP. You can find a full list
of assigned multicast addresses at www.iana.org/assignments/multicast-addresses.
 
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