Rockwell Automation T8xxx Trusted Communications Interface - Manual (Page 18)

Trusted
TM
Communication Interface T8151B
Issue 21 Apr 10 PD-T8151B 18
mask defines the portion of the whole IP address which is the network address. Devices on one
network address cannot communicate to devices on other networks without a gateway.
The Ethernet port IP addresses for the two ports on a communication interface should be set to
separate networks, i.e. the IP addresses covered by the ‘Subnet Mask’ should be different. If port 1
and 2 are on the same network, only one will be able to communicate.
Examples:
Subnet mask = 255.255.255.0 in decimal
This equals 11111111.11111111.11111111.00000000 in binary. The 1’s in the subnet mask show
where the network address portion is, i.e. any number in the first three places.
This allocates all of the first three numbers in the IP address to the network address. So IP addresses
1.2.3.123 and 1.2.3.231 are on the same network, but 1.2.4.123 is on network 1.2.4.x.
Subnet mask = 255.255.128.0
This equals 11111111.11111111.10000000.00000000 in binary.
This allocates the first two numbers in the IP address and the highest bit of the next number to the
network address. So 1.2.128.1 and 1.2.129.1 are on the same network but 1.2.127.1 is not.
3.3.2. Default Gateway
The Default Gateway is the address of a device that allows access to other networks. Since the two
ports must be on different networks, only one of the ports can be on the same network as the gateway,
and so only this port can use the gateway. The gateway allows the port to communicate with IP
addresses outside its own network, e.g. across a site LAN or even onto the internet, perhaps to allow
remote diagnostics.
For example, a Communication Interface ethernet port is connected to network 192.200.11.x. (The
subnet mask is 255.255.255.0). The gateway on this network is 192.200.11.1. The Communication
Interface ethernet port is assigned an address of 192.200.11.202. This allows it to communicate to
other devices on the 192.200.11.x network. In order to communicate with a device on another network,
e.g. 192.200.81.145, the Default Gateway setting must match the network gateway address of
192.200.11.1. The remote device will then be able to transmit and receive packets from the
Communication Interface.