Subnet Mask

Networks are far too large to allow all traffic to flood all parts of a given network.   Missouri State University's network currently (11-11-1998) consists of over 5,500 devices on three campuses and the Internet has countless number of stations.  These networks must, therefore, be broken down into smaller, more manageable parts.  These parts are called, logically enough, subnets.  Communications destine for a specific subnet will not compete with bandwidth on other subnets unless it is required to reach the specific subnet in question.  For example, traffic from a Web server in Denver to a New Hall user will never show up on Hammon's House's network.

The IP (Internet Protocol) address uniquely identifies a specific station and it consists of the network and subnet portion and the node or station portion.  All users on a given subnet will have the same net and subnet number, but will have a unique station number.  Which portion of the address identifies the net and subnet and which portion identifies the station are left up to the networking administrators to determine.   The line is drawn to best fit the physical circumstances and to conserve scarce Internet addresses.  This line is drawn using a Subnet Mask.

The Subnet Mask is a bit pattern which is used to mask off the network portions from the station portions of the address.  If your station is using the wrong subnet mask for the network to which you are attached, then it will not be able to correctly identify all users on that subnet and many users could be unreachable by your .computer

A twenty-four bit mask of 255.255.255.0 defines 254 stations per subnet and addresses look something like 146.7.25.X where "X" is a station number from 1 to 254.   (Station address 0 and 255 are reserved for broadcast applications.)  A 23 bit mask of 255.255.254.0 defines 510 stations per subnet and looks something like 146.7.72.X and 146.7.73.Y where X and Y are station numbers.  Note addresses 146.7.72.1 and 146.7.73.1 are two different stations on the same subnet.  Since IP addresses are decimal representations of eight bits of the 32 bit address, it is more confusing to use other than 24 bit masks.

Missouri State University currently uses the subnet mask 255.255.255.0 for backbone segments and for non-VLANs and 255.255.254.0 for VLAN applications.   Our links into MORENet, however, only require a small number of stations per wide area link (WAN) and use a subnet mask of 255.255.255.252 for it's interfaces.