SM-Ethernet User Guide
21
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5.6.1
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.
5.6.2
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.
5.6.3
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.
5.6.4
Class D & E addresses
These addresses are reserved for multicasting and experimental use.
5.7
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.
5.7.1
The IP address
The IP address is made up from four 8 bit decimal numbers (
octets
) and is written as
follows:
w.x.y.z
for example192.168.0.1 (
class c
)
5.7.2
The subnet mask
The subnet mask defines what part of the address constitutes the subnet within the IP
address and what part of the address constitutes the host address. The subnet mask is
bit-wise ANDed with the address to give the subnet to which the host belongs. A typical
class C subnet mask would be 255.255.255.0, this may alternatively be written as ‘/24’
as in the example below, showing an IP address of 192.168.0.1 with a subnet mask of
255.255.255.0. This alternative notation indicates the number of bits representing the
subnet part of the address, starting from the most significant bit.
Alternative subnet mask notation:
192.168.0.1 /24
5.7.3
Completing the address
To determine which part of the address constitutes the network address and which part
constitutes the node address, the IP address is bit-wise ANDed with the subnet mask.
Figure 5-1 shows how the IP address and subnet mask are used to determine the
subnet address and the host address.
Summary of Contents for SM-Ethernet
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