GE GFK-1541B manual Gateways, Networks Connected by a Gateway

6

Gateways

Gateways (also known as routers) connect individual physical networks into a system of networks. When a node needs to communicate with a node on another physical network, a gateway transfers the data between the two networks.

Networks Connected by a Gateway

The following example shows Gateway G connecting Network 1 with Network 2.

A

a45405

172.16.0.1

Network 1

172.16.0.2

G Gateway

B

C

172.17.0.3

172.17.0.1

172.17.0.2

Network 2

When host B with IP address 172.17.0.1 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 (via ARP) 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 netids 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

172.17.0.3.This address would be configured in the Ethernet Interface’s module configuration for PLC B as its default gateway address.

Note that the gateway has two IP addresses (172.16.0.2 and 172.17.0.3). 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 an IP address with a netid matching its own.

6-4 TCP/IP Ethernet Communications for the Series 90™ PLC User's Manual– May 2002 GFK-1541B