Allen-Bradley ControlLogix 1756-ENBT, User Manual

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Publication 1756-UM050A-EN-P - December 2001
3-4 Before You Configure Your Module
For more information on Internet addressing, see Comer, Douglas E;
Internetworking with TCP-IP, Volume 1: Protocols and Architecture;
Englewood Cliffs, N.J.: Prentice-Hall, 1990.
Gateways
A gateway connects individual physical networks into a system of
networks. When a node needs to communicate with a node on
another network, a gateway transfers the data between the two
networks. The following figure shows gateway G connecting Network
1 with Network 2.
When host B with IP address 128.2.0.1 communicates with host C, it
knows from C’s IP address that C is on the same network. In an
Ethernet environment, B can then resolve C’s IP address to a MAC
address and communicate with C directly.
When host B communicates with host A, it knows from A’s IP address
that A is on another network (the net IDs are different). In order to
send data to A, B must have the IP address of the gateway connecting
the two networks. In this example, the gateway’s IP address on
Network 2 is 128.2.0.3.
The gateway has two IP addresses (128.1.0.2 and 128.2.0.3). The first
must be used by hosts on Network 1 and the second must be used by
hosts on Network 2. To be usable, a host’s gateway must be addressed
using a net ID matching its own.
TIP
Contact your network administrator or the Network
Information Center for a unique fixed IP address to
assign to your module.
Tip
128.1.0.1
128.2.0.1 128.2.0.2
128.2.0.3
128.1.0.2
A
B C
G
Network 1
Network 2