Which subnet masks are allowed for a TCP/IP configuration of a PCD?

FAQ #100318

There have been some limitations according the IP norm that allowed only certain combinations of IP adresses and subnet masks (only classful networks with subnetting were allowed). However, these limitations are (or will be) removed in recent firmware versions according the Classless Internet Domain Routing (CIDR).


The IP stack of our PCD controllers (PCD3 and PCD7.F655) was only supporting "classful networks with subnetting" according to the official IP standards. As result only IP address/subnet masks that conformed to the "classul networks with subnetting" were allowed in certain firmware versions of the PCD3 and the PCD7.F655. If the address/subnetmask was not conform to the rules (because e.g. supernetting was applied), the communication didn't work.
We are now introducing the "Classless Internet Domain Routing" CIDR to our stack which provides increased flexibility when dividing ranges of IP addresses into separate networks.


Classful networks
In classful networks (without subnetting) it is possible deriving the subnet mask based on the IP address accoring to the table below.


IP address range  

Network class Subnet mask to A to B to C

Further on, subnetting can be introduced by extending the "network portion" by using a part of the "host portion" (the right side of the subnet mask, in the classful network filled with "0"). Subnetting works with all PCD firmwares but supernetting does not (see example below).
The PCD3 firmwares older than the one mentioned in the table below does not allow class A or B subnet masks if the IP adresses are not in the defined range (because this would be supernetting).

Classless networks
For the "Classless Internet Domain Routing" CIDR the network classes mentioned above aren't relevant for the subnet mask any more. Therefore it is possible to define any subnet mask (subnetting and supernetting is allowed). Of course the network will only work properly if all the devices are configured correctly.
When in doubt about the required subnet mask for your PCD, please contact your network adminstrator.


Firmware versions introducing CIDR

System Firmware introducing CIDR
PCD3.Mxxxx $31
PCD7.F655 043


Related Files:


PCD3 / Mxxx

PCD7 / Fxxx

Communication / Ether-S-Bus

Last update: 28.05.2015 08:30

First release: 11.02.2005 08:13

Views: 4432

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