Netmask (NM) setting
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Description (see setting description format info here)
Netmask defines the boundaries of a local subnet. When establishing an outgoing connection to the destination network host at current Destination IP-address (DI) [setting/ parameter/ instruction] the DS compares this address with the Netmask and its own IP-address (IP) to determine if the destination is on the local or foreign subnet. The DS will (attempt to) connect directly to the current Destination IP-address (DI) if the destination host is found to reside on the local subnet or to the Gateway IP-address (GI) if the destination is found to reside on a foreign subnet.
Comparison is done as follows:
Example: supposing, the IP-address (IP) of the DS is 192.168.100.40 (C0.A8.64.28 in HEX representation), current Destination IP-address (DI) is 192.168.100.90 (C0.A8.64.5A in HEX) and the Netmask is 255.255.255.0 (FF.FF.FF.00 in HEX). Then:
Here is another way of explaining how the Netmask works. When printed in binary representation, the Netmask always consists of a number of 1s on the left and the number of 0s on the right (for the example above the Netmask value is 1111111. 11111111. 11111111. 00000000). Positions with 1s (left side) represent the part in which the current Destination IP-address (DI) must match the IP-address (IP) of the DS to be considered local. Positions with 0s (right side) represent the range of IP-addresses belonging to the same subnet. If the Netmask is 255.255.255.0 then any IP-address that starts with 255.255.255 will be on the same subnet.
When DHCP is activated (DHCP (DH) setting is 1(enabled)) the Gateway IP-address obtained from the DHCP server is saved into this setting thus overwriting older value that might have been set before. This only happens when the DHCP server is configured to provide netmask data.