1-7
Section
IP Addresses
9
Class
Number of networks
Number of hosts
Class A
Small
2
24
– 2 max. (16,777,214 max.)
Class B
Medium
2
16
– 2 max. (65.534 max.)
Class C
Large
2
8
– 2 max. (254 max.)
IP addresses are represented by the decimal equivalent of each of the four oc-
tets in the 32-bit address, each separated by a period. For example, the binary
address
10000010 00111010 00010001 00100000
would be repre-
sented as 130.58.17.32.
Note The same network number must be set for every node on the same Ethernet
network.
1-7-2 Allocating IP Addresses
IP (Internet Protocol) is a standard communications protocol used throughout
the world and is designed to enable communications between any Ethernet
nodes regardless of the networks on which they exist. To achieve this, network
numbers are allocated by the Network Solutions, InterNIC Registration Ser-
vices, to ensure that all Ethernet networks have unique numbers regardless of
where they exist. The local system administrator is left the responsibility of allo-
cating unique host numbers locally. You therefore should obtain a network num-
ber from the InterNIC Registration Services to ensure uniqueness and allow for
future network expansions if required.
1-7-3 IP Address Settings
An IP address must be set for the Ethernet Unit before Ethernet communications
can proceed. The IP address is set for each Ethernet Unit by the rotary switch on
the back of the Unit. For more details, refer to 3-4-3 Setting the Local IP Address.
1-7-4 Subnet Masks
Operation and management of a network may become very difficult if too many
nodes are connected on a single network or if a single organization has to man-
age too many network numbers. It can therefore be convenient to divide a single
network up into several subnetworks by using part of the host number as a sub-
net number. Internally the network can be treated as a number of subnetworks,
but from the outside it acts as a single network and uses only a single network
number.
To establish subnetworks, the host number in the IP address is divided into a
subnet number and a host number by using a setting called the subnet mask.
The subnet mask indicates which part of the host number is to be used as the
subnet number. The user must first determine the number of bits of the host
number to be used as the subnet number and then set the subnet mask accord-
ingly. All bits in the subnet mask that correspond to the bits in the IP address
used either as the network number or subnet number are set to “1” and the re-
maining bits, which will correspond to the bits in the IP address actually used for
the host number, are set to “0”.
The following example shows the subnet mask for an 8-bit subnet number used
in a class-B IP address. This subnet mask is structured as follows:
This would thus be a class-B IP address masked as a class-C IP address, i.e.,
externally it would be allocated a class-B IP address but internally it can be ad-
dressed using class-C IP addresses.
Subnet mask:
11111111 11111111 11111111 00000000
(FFFFFF00)
It is only necessary to set subnet masks if subnetworks are used. If a subnet
mask is not set by the user, a default mask will be set automatically according to
the IP address class to indicate that the entire host number will be used as the
host number, i.e., no bits will be assigned for use as the subnet number.
