18 SI-Ethernet User Guide
Issue: 3
5.5 Class types
IP addresses are grouped into ranges called classes, each class has a specific set of addresses
and has a typical situation where it is used.
When selecting the class of IP address required, consideration must be given to how many subnets
you need, how many hosts are required and if you will need a public (worldwide) or a private (local)
addressing scheme. Table 5-1 shows an overview of how the class types are defined and Table 5-2
shows how each class separates the subnet and host ID.
Table 5-1 Subnets and hosts supported by class type
Table 5-2 Address components
5.5.1 Class A addresses
A class A address only uses the first octet to represent the subnet, the remaining octets are used to
represent the host id. These addresses are intended for large organisations such as universities
and the military. These addresses must be requested from the governing body (InterNIC) when
using them publicly (on the Internet) to avoid duplication.
5.5.2 Class B addresses
A class B address uses the first two octets to represent the subnet, the remaining octets are used to
represent the host id. These addresses are intended for medium to large size networks. These
addresses must be requested from the governing body (InterNIC) when using them publicly (on the
Internet) to avoid duplication. Class B addresses are generally used on public or private networks.
5.5.3 Class C addresses
Class C addresses use the first 3 octets as the subnet address and the remaining octet as the host
id. A class C address is normally used on a private network only, due to the restriction on the
number of hosts on the network. Class C addresses will not be routed onto the Internet.
5.5.4 Class D & E addresses
These addresses are reserved for multicasting and experimental use.
5.6 Generating the complete address
A complete IP address consists of an IP address and a subnet mask, these two numbers are
required to allow communication on Ethernet using TCP/IP.
Address Class First Octet Decimal
Range
Number of
Subnets
Number of Hosts
A 1-126.x.y.z 126 16,777,214
B 128-191.x.y.z 16,382 65,534
C 192-223.x.y.z 2,097,150 254
Address Class IP Address Subnet Component Host Component
A w.x.y.z w x.y.z
B w.x.y.z w.x y.z
C w.x.y.z w.x.y z
Using the subnet mask it is possible to modify the IP addressing such that the ratio of
subnets and host addresses may be changed. This gives you the facility to “adjust”
standard classes to suit your specific requirements.
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