User's Manual

SPEEDLAN 9000 Installation and Operation User Guide
Basics of IP Addressing 5-5
Default Subnet masks
• Class A: 255.0.0.0
• Class B: 255.255.0.0
• Class C: 255.255.255.0
Note: Subnet mask is bolded.
What is a Subnet?
Subnetting allows you to create multiple networks within one Class A, B, or C network. Each data
link (octet) contains its own unique identifier also known as the subnet. Also, each node on the same
data link must belong on the same subnet as well.
What is a Subnet Mask?
A subnet mask allows you to mask section(s) (depending on the class specified) of the octets in the
network address. Each octet used in the subnet mask is assigned to a data link. The leftover octet(s)
are assigned to the remaining nodes.
For more information on subnetting, see the example below and Diagram of Subnetting a Network,
page 5-6.
Example of Subnetting:
For example, a Class C network (255.255.255.0) contains three masked octets (255.255.255). The
last octet (0) is leftover for remaining nodes (i.e., computers).
If Unit/Router D is reading IP Addresses 195.172.3.1 (let's call this IP Address 1) and 195.172.3.64
(let's call this IP Address 2) on this Class C network, it would send IP Address 1 to Subnet A and IP
Address 2 to Subnet B. The remaining nodes in each subnet (A through D) on this network can
contain up to 254 pieces of network equipment (computers, printers, fax machines, bridges or units/
routers, etc.).