User manual

Publication ENET-UM001I-EN-P - January 2010 221
EtherNet/IP Network Overview Appendix D
When host B communicates with host A, it knows from A’s IP address that A
is on another network because the network IDs differ. 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). Network
1 hosts
must use the first IP address, and Network 2 hosts must use the second IP
address. To be usable, a host’s gateway IP address must match its own net ID.
Subnet Mask
Subnet addressing is an extension of the IP address scheme that 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.
Take Network 2 (a Class B network) in the previous example and add another
p
h
ysical network. Selecting this subnet mask would add two additional net ID
bits allowing for four physical networks.
11111111 11111111 11000000 00000000 = 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 physical network.
New Configuration
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