GE GFK-1541B manual IP Addressing, IP Address Format for Network Classes A, B, C

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IP Addressing

Each TCP/IP node on a network must have a unique IP address. The TCP/IP Ethernet Interface is such a node, as is a PC running TCP/IP. There may be other nodes on the network that are not involved with communications to the PLCs, but no matter what their function, each TCP/IP node must have its own IP address. It is the IP address that identifies each node on the IP network (or system of connected networks). The term “host” is often used to identify a node on a network.

IP Address Format for Network Classes A, B, C

The IP address is 32 bits long and has a netid part and a hostid part. Each network is a Class A, Class B or Class C network. The class of a network determines how an IP address is formatted and is based on the number of bits in the netid part of the IP address.

	0 1			8		16	24	31
Class A	0		netid				hostid
	0 1			8		16	24	31
Class B	1	0		netid			hostid
	0 1		2	8		16	24	31
Class C	1	1	0		netid			hostid

In general, the netid part is assigned by the Internet authorities and the hostid part is assigned by your local network administrator. The class of network determines the number of hosts that can be supported. A Class A network can support 224-2 (16,777,214) hosts, Class B, 216-2 (65,534) hosts, and Class C, 28-2 (254) hosts. The minus 2 refers to host numbers reserved for the network itself and the local broadcast.

Each node on the same physical network must have an IP address of the same class and must have the same netid. Each node on the same physical network must have a different hostid thus giving it a unique IP address.

IP addresses are written in “dotted-decimal” format as four decimal integers (0-255) separated by periods where each integer gives the value of one byte of the IP address. For example, the 32-bit IP address

00001010 00000000 00000000 00000001

is written as

10.0.0.1

One can determine the class of an IP address by examining the first integer in its dotted-decimal IP address and comparing with the range of values in the following table.

Range of first integer	Class
0 - 127	A
128 - 191	B
192 - 223	C
224-239	D (Reserved for Multicast Use)
240 - 255	E (Reserved for Experimental Use)

6-2 TCP/IP Ethernet Communications for the Series 90™ PLC User's Manual– May 2002 GFK-1541B