Specifications

RSLinx – Training Guide A - 9
7) What are the specifications for a Class A, Class B, and Class C network?
In a Standard Class A network, the first octet is reserved for the
Network Address, and the remaining three octets are reserved for the
Node Addresses. The Class A format is:
Network.Node.Node.Node
To determine the valid number of Network Addresses, recall that a
Class A network has a first octet with the leading bit pattern 0. This
leaves only 7-bits available, from the first octet, for Network
Addresses. Converting this binary number to decimal gives a total of
128 (0-127 decimal) possible Network Addresses. A Network Address
of 0 is invalid and cannot be used. Also, a Network Address of 127 is
invalid and cannot be used. Thus, the only valid Class A Network
Addresses available will be between 1-126 decimal. This means that
any TCP/IP address that begins with a number between 1 and 126 is a
Class A Network.
To determine the valid number of Node Addresses, use the remaining
three octets for addressing the hosts. Converting this 24-bit binary
number to decimal gives a total of 16,777,216 possible Node
Addresses. A Node Address of all zeros and all ones is invalid. Thus,
this leaves a total of 16,777,214 possible Node Addresses available
for a Standard Class A Network.
In a Standard Class B network, the first two octets are reserved for the
Network Address, while the next two octets are reserved for the Node
Addresses. The Class B format is: