What is a Subnet Mask?
The subnet mask is the portion of an IP address that defines the network, as opposed to identifying a particular computer. For example, a computer with an IP address of 192.168.1.5 might have a subnet mask of 255.255.255.0.
The portion of the address that reads 192.168.1 is the network address (also known as the network ID), and the .5 is the address of the specific machine on that network (also known as the host ID).
The subnet mask explains that the first three numbers in a dotted-decimal address (for example: 192.168.1) are the network and that the last number denotes a specific computer on that network.
Once a packet has arrived at an organization's IP gateway or a connection point with its unique network number, it can be routed within the organization's internal gateways using the subnet number. For more information refer to Definition of an IP Gateway.
The router knows which bits to look at (and which not to look at) by looking at a subnet mask. A mask is simply a screen of numbers that tells you which numbers to look at underneath. In a binary mask, a "1" over a number says "Look at the number underneath"; a "0" says "Don't look." Using a mask saves the router from having to handle the entire 32-bit address; it can simply look at the bits selected by the mask. The division of an organization's network into subnets allows it to be connected to the Internet with a single shared network address.
Using the previous example (which is a very typical case), the combined network number and subnet number occupy 24 bits or three of the quads. The appropriate subnet mask carried along with the packet would be 255.255.255.0, or a string of all 1's for the first three quads (telling the router to look at these) and 0's for the host number (which the router doesn't need to look at). Subnet masking allows routers to move packets more quickly.
Note: In a Lexmark context, the subnet is one third of the IP scheme (IP, Netmask, Gateway) that you need to know before you can begin your printer wireless or network setup. Generally, you can find out what gateway to use on your printer by identifying the gateway being used by your computer's network interface card (see below). Otherwise, you will need to contact your network administrator. If you have no network administrator, consult the individual(s) who set up your network.