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B-2 User’s Reference Guide
IP addresses indicate both the identity of the network and the identity of the individual host on the network. The
number of bits used for the network number and the number of bits used for the host number can vary, as long
as certain rules are followed. The local network manager assigns IP host numbers to individual machines.
IP addresses are maintained and assigned by the InterNIC, a quasi-governmental organization now increasingly
under the auspices of private industry.
Note: It’s very common for an organization to obtain an IP address from a third party, usually an Internet
service provider (ISP). ISPs usually issue an IP address when they are contracted to provide Internet access
services.
The InterNIC (the NIC stands for Network Information Center) divides IP addresses into several classes.
Classes A, B, and C are assigned to organizations that request addresses. In Class A networks, the first byte of
an IP address is reserved for the network portion of the address. Class B networks reserve the first two bytes
of an IP address for the network address. Class C networks reserve the first three bytes of an IP address for the
network address. In all cases, a network manager can decide to use subnetting to assign even more bits to the
network portion of the IP address, but never less than the class requires. The following section gives more
information on subnetting.
Class A networks have a small number of possible network numbers, but a large number of possible host
numbers. Conversely, Class C networks have a small number of possible host numbers, but a large number of
possible network numbers. Thus, the InterNIC assigns Class A addresses to large organizations that have very
large numbers of IP hosts, while smaller organizations, with fewer hosts, get Class B or Class C addresses. You
can tell the various classes apart by the value of the first (or high-order) byte. Class A networks use values from
1 to 127, Class B networks use values from 128 to 191, and Class C networks use values from 192 to 223.
The following table summarizes some of the differences between Class A, B, and C networks.
SS
SS
uu
uu
bb
bb
nn
nn
ee
ee
tt
tt
ss
ss
aa
aa
nn
nn
dd
dd
ss
ss
uu
uu
bb
bb
nn
nn
ee
ee
tt
tt
mm
mm
aa
aa
ss
ss
kk
kk
ss
ss
Often an entire organization is assigned only one IP network number. If the organization has several IP networks
connected together with IP routers, the network manager can use subnetting to distinguish between these
networks, even though they all use the same network number. Each physical network becomes a subnet with a
unique subnet number.
Subnet numbers appear within IP addresses, along with network numbers and host numbers. Since an IP
address is always 32 bits long, using subnet numbers means either the network number or the host numbers
must use fewer bits in order to leave room for the subnet numbers. Since the InterNIC assigns the network
number proper, it should not change, so the subnet numbers must be created out of bits that would otherwise
be part of the host numbers.
Class First byte
Number of
networks
possible per
class
Number of
hosts
possible per
network
Format of address
(without subnetting)
Example
A 1–127 127 16,777,214 net.host.host.host 97.3.14.250
B 128–191 16,384 65,534 net.net.host.host 140.100.10.11
C 192–223 2,097,152 254 net.net.net.host 197.204.13.7