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Disclaimer: This document is written in the hope that you can utilize for your own education to gain knowledge of PLC systems (should you decide to utilize this document). Although I believe the information in this document to be accurate, it is YOUR responsibility to verify this information before implementing it in any way, especially when damage to personnel or equipment could result. By continuing to read this document, you agree to hold no one who writes, modifies, or distributes this document liable in any way (even negligence). Due to the wide variety of plant applications, some of the examples in this document may be prohibited at your location, or could cause damage to equipment, or harm personnel.
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Ethernet Addressing


Every device on the same Ethernet network must have a unique address. Allen Bradley PLC's currently use the IP (Internet protocol) addressing scheme (version 4). This is the addressing scheme discussed in this document.


Examples of Ethernet devices might be a Personal Computer (PC), a 1756-ENBT module, a 1794-AENT flex adapter, a printer with built-in print server capabilities, and plant servers for data storage and processing.


To see what the IP address is of your Windows NT, 2000, or XP machine type:

ipconfig /all

at the command prompt. Here is the result:





For Unix/Linux systems, the command /sbin/ifconfig will product similar results.


In this example, my PC has the IP address of 192.168.0.101. No other machine on the same network will have this same IP address, nor should you attempt to assign the same address to another device on the network. If this happens, one of the devices will not be seen.


Each segment of the IP address is called an OCTET. All IP addresses in the IPv4 addressing scheme are made up of 4 octets. Each octet is an 8 bit unsigned integer.





Subnet Mask


Notice the Subnet Mask: 255.255.255.0. The purpose of the subnet mask is to identify which part of the IP address is the network address, and which part of the IP address is the host or terminal on the network. To understand how the subnet mask works, it has to be broken down into a binary format... Look at the IP address in Binary:



11000000 . 10101000 . 00000000 . 01100101 = 192.168.0.101


Now look at the subnet mask:


11111111 . 11111111 . 11111111 . 00000000 = 255.255.255.0


Anywhere a 1 exists in the subnet mask, that bit of the IP address is viewed as the NETWORK part of the address.... Lets see what bits are passed:


11000000 . 10101000 . 00000000 . 01100101 = 192.168.0.101

11111111 . 11111111 . 11111111 . 00000000 = 255.255.255.0

11000000 . 10101000 . 00000000 . dont pass = 192.168.0.X


Wherever there was a 1 in the subnet, we passed that bit of the IP address as part of the network address. Therefore, we would say the network address for this machine is 192.168.0.X. Every device on this network must have an IP address that starts with 192.168.0. and X is the terminal address on the network.


For example: Devices with these two IP addresses can communicate with each other directly without going through a router:


192.168.0.3 and 192.168.0.200 with subnet 255.255.255.0


These two devices cannot communicate with each other directly:


192.168.1.3 and 192.168.3.200 with subnet 255.255.255.0


However if the subnet mask was changed:


192.168.1.3 and 192.168.3.200 with subnet 255.255.0.0

These two devices will communicate with each other because the network address is only made up of the first two octets, 192.168.X.X... since the network address is the same for the two devices, they will communicate directly.



Subnet Mask Exercise:

Indicate whether or not the following devices can communicate with each other directly:




  1. 192.168.0.5 and 192.168.0.6 with subnet mask of 255.255.255.0 YES NO


  1. 192.168.1.5 and 192.168.0.6 with subnet mask of 255.255.255.0 YES NO


  1. 192.168.1.5 and 192.168.0.6 with subnet mask of 255.255.0.0 YES NO


  1. 10.1.1.5 and 10.1.1.6 with subnet mask of 255.255.255.0 YES NO


  1. 10.1.1.5 and 10.1.1.6 with subnet mask of 255.0.0.0 YES NO


  1. 10.2.5.5 and 10.1.1.6 with subnet mask of 255.255.255.0 YES NO


  1. 10.2.5.5 and 10.1.1.6 with subnet mask of 255.0.0.0 YES NO


  1. 10.2.5.5 and 10.1.1.6 with subnet mask of 255.255.0.0 YES NO


  1. 192.168.0.1 and 10.1.1.6 with subnet mask of 255.0.0.0 YES NO


  1. 10.2.1.5 and 10.1.1.6 with subnet mask of 255.0.0.0 YES NO


Other Terms:



GATEWAY

The gateway address is the IP address of a server or hardware router that connects you to other networks such as the Internet.



DNS (Primary and Secondary)

The DNS server (Domain Name Server) resolves host names into IP addresses. When you enter an address such as Yahoo.com into your web browser, your PC does not understand where to go. It must ask the DNS server to look up the IP address of a given host names. Host names are for humans to understand. Computers understand IP addresses.



DHCP

Dynamic Host Configuration Protocol – When a device such as a computer is configured to use DHCP, A DHCP server should be available on the network. As soon as the device connects to the network, it will ask the DHCP server to automatically assign an IP address, subnet mask, DNS servers, Gateway address, etc. This address is dynamic, so the device could get a different IP address each time it's connected.



BootP

Bootstrap Protocol – Similar to DHCP, except a BootP device will get the same IP address every time it connects. The BootP server has a list of hardware addresses, and IP addresses that belong to each device. When a device (such as a PLC) connects to the network, it will give the BootP server it's hardware address. The server will then look up the hardware address in a list, and see which IP address belongs to the PLC. The BootP server will then return the IP address (and other information such as the subnet mask) to be used by the device.