Rockwell Automation 1747-AENTR SLC 500 EtherNet/IP Adapter User Manual - Manual (Page 21)

Rockwell Automation Publication 1747-UM076C-EN-E - January 2013 15
Configure the Adapter for Your EtherNet/IP Network Chapter 3
When a node needs to communicate with a node on another network, a gateway
transfers the data between the two networks. The 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 into a hardware address (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 network IDs are different). In order to send data to A, B
must use 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 gateway of a host must be addressed using a network ID matching
its own.
Subnet Mask
The subnet mask is used for splitting IP networks into a series of subgroups, or
subnets. The mask is a binary pattern that is matched up with the IP address to
turn part of the Host ID address field into a field for subnets.
EXAMPLE Take Network 2 (a Class B network) in the previous example and add
another physical network. Selecting the following subnet mask would
add two additional network ID bits, allowing for four physical networks:
11111111 11111111 11000000 00000000 = 255.255.192.0
Network 1
Network 2
128.1.0.1
128.2.0.1 128.2.0.2 128.2.0.3
128.1.0.2
A
B
G
C
These two bits of the Host ID are used to
extend the netdwork ID.