Subnet Mask - Allen-Bradley Rockwell Automation 1734-AENT User Manual

18 Configure the Adapter for Your EtherNet/IP Network
Publication 1734-UM011D-EN-P - May 2011
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.
A
128.1.0.1
B
C
128.2.0.1
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 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.
Take Network 2 (a Class B network) in the previous
EXAMPLE
example and add another physical network. Selecting the
following subnet mask would add two additional net ID
bits, allowing for four physical networks:
11111111 11111111 11000000 00000000 = 255.255.192.0
These two bits of the Host ID are used to
extend the net ID.
Two bits of the Class B host ID are 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 physical network.
Network 1
128.1.0.2
G
128.2.0.3
128.2.0.2
Network 2