IP addresses have two discrete components (network portion and node portion), but do not provide a visual indicator as to where the network portion ends and the node portion starts. This apparent shortcoming is actually a key part of TCP/IP’s design. The 32-bit addresses can be split almost anywhere within the 32-bits, allowing for many networks with only a few nodes, or for a few networks with many thousands of nodes. This flexibility makes TCP/IP equally usable on small and huge networks.
In order to allow for flexible addressing, a "mask" is used to determine which portion of the address refers to the network, and which portion refers to the node. The mask is also a 32-bit number, divided into four 8-bit fields, similar to the address itself. Wherever a bit in the mask is "on", the corresponding bit in the IP address is to be used as part of the network address. Wherever a bit in the mask is "off", the corresponding bit in the IP address is to be used as part of the node address.
For example, the IP address of 192.168.1.10 has a mask of 255.255.255.0. The first 24 bits of the mask are all "on" indicating that the first 24 bits of the IP address are to be used for the network number, while the last 8 bits are to be used for the node number.
There are three common sizes of TCP/IP networks, commonly referred to as "classes" of addresses. "Class A" networks have a mask of 255.0.0.0, while "Class B" networks have a mask of 255.255.0.0, and "Class C" networks have a mask of 255.255.255.0. When addresses are assigned by the Internet authorities, they are only assigned in one of these three classes.
However, once an address block has been assigned to you, the block can be split into as many sub-networks as you wish by changing the subnet mask. This allows an organization to split a single Class C network into smaller groups of devices so that small LAN segments do not consume an entire Class C network. It also allows Internet Service Providers to split a single Class B network into many smaller networks, which can then be assigned to organizations that are using the Internet Service Provider for Internet connectivity.
You may need to discuss this subject with other system administrators, consultants, or service providers in order to determine the correct subnet mask to use on this system.
The subnet mask that you choose to assign this device will be determined by several factors:
For more information about subnet masks, refer to section B.3.1.4 Subnet masks.
STEP: To continue to the next step in the configuration process, go to section 2.2.3.3 Choose a Routing Service.
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