User guide

MDX-ETHERNET User Guide 15
Issue : a www.leroy-somer.com
5.4 Addressing etiquette
The following list details some points that should be considered when selecting
addresses :
• Reserve address space : Ensure you have enough reserve address space
on your chosen addressing scheme to allow for future expansion.
• Uniqueness : Ensure your addresses are unique, every device on a subnet
must have a unique address.
• Avoid reserved addresses : For example the address 127.0.0.1 is reserved
as the loop back address.
• Broadcast and system addresses : The highest and lowest host address on
a subnet are reserve addresses.
• Use a system : Have a scheme for assigning your addresses, for example
typically servers may have a low IP address and routers a high IP address. It
is not necessary to allocate consecutive IP addresses so it is possible to
reserve ranges for specific uses such as servers, work stations or routers.
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 organizations
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.
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.
NOTE