Subnet Mask

A Subnet Mask is the method used to define the limits of a range of IP Addresses used in a specific network (see IP Address in this Glossary for more information).

Subnet Masks are a string of numbers organized into four sets called octets. Each octet is separated by a period ( . ). While a Subnet Mask resembles an IP Address, it is completely different and uses a different range of number values. There are standard and custom Subnet Masks.

Customizing Subnet Masks is a subject beyond the scope of this glossary and is not necessary to know in order to configure and use networking on the IT-100. Standard Subnet Masks come in three classes and define three different sizes of networks from largest to smallest. Class A is 255.0.0.0.  Class B is 255.255.0.0. Class C is 255.255.255.0. The IT-100's LAN and Wireless interfaces both use the Class C Subnet Mask of 255.255.255.0. This limits the number of usable IP addresses to 252 (actually 253, but one address is assigned by default to the IT-100's interface for that network).

You should never have to configure Subnet Masks on the IT-100 but if you manually configure a computer or other networked device for one of the IT-100's networks, you must make sure you use the 255.255.255.0 subnet mask for those devices. In order for the IT-100 to connect to the Internet, your ISP will most likely assign an IP address and Subnet Mask to the WAN port of the device automatically. They may use a different Subnet Mask than the one used by the LAN and Wireless ports but unless they request that you manually configure the WAN port's address and Subnet Mask, you will be unaware of this information. In some of the IT-100's documentation, a Subnet Mask is also referred to as a Netmask.