Rockwell Automation 1756-NEX Nexus Module - Manual (Page 33)

About the NeXUS Network 3-7
Publication 1756-UM524B-EN-P - December 2008
Subnet Mask
Subnet addressing is an extension of the IP address scheme that allows a site to use a
single net ID for multiple physical networks. Routing outside of the site continues by
dividing the IP address into a net ID and a host ID via the class. Inside a site, the subnet
mask is used to redivide the IP address into a custom net ID portion and host ID portion.
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 net ID bits
allowing for four physical networks:
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 physical network.
The new configuration is:
A second network with Hosts D and E has been added. Gateway G2 connects Network 2.1
with Network 2.2. Hosts D and E will use Gateway G2 to communicate with hosts not on
Network 2.2. Hosts B and C will use Gateway G to communicate with hosts not on
Network 2.1. When B is communicating with D, G (the configured Gateway for B) will
route the data from B to D through G2.
EXAMPLE
11111111 11111111 11000000 00000000 = 255.255.192.0
128.1.0.1
128.2.64.1 128.2.64.2
128.2.64.3
128.1.0.2
A
B C
G
Network 1
Network 2.1
128.2.128.1 128.2.128.2
128.2.128.3 D E
G2
Network 2.2