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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 then resolves C's IP address into a hardware address (MAC
address) and communicates 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.
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 network. Selecting the following subnet mask would add two
additional net ID bits, allowing for four logical networks:
11111111 11111111 11000000 00000001 = 255.255.192.0
Two bits of the Class B host ID have been used to extend the net ID. Each
unique combination of bits in the part of the Host ID where subnet mask bits
are 1 specifies a different logical network.
The new configuration is:
A
128.1.0.1
B
128.2.64.1
D
128.2.128.1
A second network with Hosts D and E was added. Gateway G2 connects
Network 2.1 with Network 2.2.
Hosts D and E use Gateway G2 to communicate with hosts not on Network 2.2.
Hosts B and C use Gateway G to communicate with hosts not on Network 2.1.
When B is communicating with D, G (the configured gateway for B) routes the
data from B to D through G2.
Rockwell Automation Publication 1732E-UM003D-EN-E - November 2021
Chapter 5
Configure the Module for Your EtherNet/IP Network
These two bits of the host ID
used to extend the net ID
Network 1
G
C
Network 2.1
G2
E
128.2.128.2
Network 2.2
128.1.0.2
128.2.64.3
128.2.128.3
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