Rockwell Automation Allen-Bradley 48CR Series, User Manual, Page: 24 / 178

24 Rockwell Automation Publication 48CR-UM001A-EN-P - November 2019
Chapter 4 Configure for EtherNet/IP Network
Gateway Address
This section applies to multi-network systems. If you have a single-network
system, go to Subnet Mask on page 24 . The gateway address is the default address
of a network, it provides one domain name and point of entry to the site.
Gateways connect individual networks into a system of networks. When a node
must communicate with a node on another network, a gateway transfers the data
between the two networks. Figure 4 shows gateway G, which connects Network
1 with Network 2.
Figure 4 - Two Network Connection with Gateway G
When host B with IP address 128.2.0.1 communicates with host C, it knows
from the IP address of C that C is on the same network. In an Ethernet
environment, B then resolves the IP address of C into a hardware address (MAC
ID) and communicates with C directly.
When host B communicates with host A, it knows from the IP address of A that
A is on another network (the net IDs are different). To send data to A, B must
have the IP address of the gateway that connects the two networks. In this
example, the IP address of the gateway on Network 2 is 128.2.0.3.
The gateway has two IP addresses (128.1.0.2 and 128.2.0.3). The hosts on
Network 1 must use the first and the hosts on Network 2 must use the second. To
be usable, the gateway of the host must be addressed using a net ID that matches
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.
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 another logical network.
A
128.1.0.1
128.1.0.2
G
128.2.0.3
B C
128.2.0.1 128.2.0.2
Network 1
Network 2
EXAMPLE Take Network 2 (a Class B network) in the previous example and add
another network. The selection of the following subnet mask would add
two additional net ID bits, which allows for four logical networks:
1111111 11111111 11000000 00000001 = 255.255.192.0
These two bits of the host ID used to extend the net ID