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GFK-2224Q January 2017 241
Chapter 13 Network Administration
This chapter discusses how devices are identified on the network and how data is routed among devices. The
main topics covered are:
▪
IP Addressing
▪
Gateways
▪
Subnets and Supernets
13.1 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
Class A 0 netid hostid
8 16 24 31
0 1
Class B 1 netid hostid 0
8 16 24 31
0 1 2
Class C 1 netid hostid 1 0
8 16 24 31
Figure 97: IP Address Format for Network Classes A, B, C
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 2
24
-2 (16,777,214) hosts, Class B, 2
16
-2 (65,534) hosts, and Class C, 2
8
-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