Rockwell Automation Ethernet Design Considerations Reference Manual - Manual (Page 18)

18 Rockwell Automation Publication ENET-RM002C-EN-P - May 2013
Chapter 1 EtherNet/IP Overview
Subnet Mask
Subnet addressing is an extension of the IP address scheme. It enables 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 IP class.
Inside a site, the subnet mask is used to redivide the IP address into a custom net
ID portion and host ID portion.
A subnet mask determines which of the 32 bits in the IP address are part of the
network ID and which are part of the unique node identification. This also
determines the size of the network or subnetwork.
Take Network 2 (a Class B network) in the previous example and add another
physical network. Selecting this subnet mask adds two additional net ID bits
providing for four physical networks.
11111111 11111111 11111111 00000000 = 255.255.255.0
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 physical network.
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 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.
128.1.0.1
128.2.64.1
128.2.64.3
128.1.0.2 Network 1
Network 2.1
128.2.128.1 128.2.128.2
128.2.128.3
128.2.64.4
128.1.0.1
128.2.64.1
128.2.128.1
128.2.64.2
128.2.128.2
128.1.0.2
128.2.64.3
128.2.128.3
A
B C
G
G2
D E
Network 2.2