HP MicroLogix 1761-NET-ENI manual Subnet Mask Auto-Detect Operation, Subnet Mask is in auto-detect

ENI Configuration (Node 248 to 254) 4-13

Subnet Mask

A subnet mask is used to interpret IP addresses when the network is divided into subnets. If your network is not divided into subnets, then leave the subnet mask at the default or allow the ENI Configuration Utility to assign a default.

The subnet mask defaults to auto-detect mode “out-of-box”. As long as the subnet mask is not manually set, the auto-detect mode follows the rules as shown below:

Table 4.4 Subnet Mask Auto-Detect Operation

When the IP Address is set to:	And the Subnet Mask is in auto-detect:
	The Subnet Mask is set to:
Out of Box: 0.0.0.0	Switch-on auto-detect
Class A address	255.0.0.0
Class B address	255.255.0.0
Class C address	255.255.255.0

Subnet Mask Auto-Detect Rules:

•Reading the subnet mask when the IP address is 0.0.0.0 returns a value of 0.0.0.0.

•When you manually configure the subnet mask, auto-detect is switched off and the configured mask is used.

•The ENI validates the configured subnet mask and if:

–The first octet is not equal to 255, the ENI returns status 0x10 and reverts to the previous mask, or

–The first octet is 255, but the remaining mask is not proper, the ENI returns status 0x10 and reverts to the previous mask.

•The definition of “proper” is that the mask must be a contiguous series of 1’s with no zeroes in between (i.e. 255.0.0.0 or 255.224.0.0 or 255.192.0.0 are valid, but 255.160.0.0 is not).

Security Mask

The Security Mask, when configured, allows you to restrict controller access to sources with IP addresses that are within some prescribed range. For example, if you wanted to restrict all message sources to be from within a company’s allocated IP address range, a Security Mask could be configured that would block any IP address outside that range. This only applies to messages to the controller. Web page access, for example, is not restricted.

Publication 1761-UM006A-EN-P - February 2001