GE PACSystems RX7i, User Manual

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Chapter 13. Network Administration
GFK-2224Q January 2017 245
13.2 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.
172.16.0.1
Network 1
172.17.0.1
172.17.0.2
172.17.0.3
Network 2
172.16.0.2
Gateway
G
A
B C
Figure 101: Gateway Connected to Two Networks
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.
13.3 Subnets and Supernets
Subnets allow a site’s network administrators to divide a large network into several smaller networks while still
presenting the overall network as one single entity to the outside world. Each of the site’s interior gateways
need only maintain the subnet numbers of other interior gateways instead of every single host on the entire
network.
PACSystems Ethernet interfaces support “supernetting,” a technique of configuring the subnet mask to allow
communication to multiple subnets. The resulting supernet is a block of contiguous subnets addressed as a
single subnet.