Subnetting involves dividing an IP address range into two or more separate ranges, called subnets. Although subnetting can be done for a variety of reasons, from departmental to geographic to political, it is usually done to simplify administration in some way. Breaking networks into smaller, easily defined subnets allows the administrator to better address data flow and security and creates a logical map that can facilitate troubleshooting in the event of a problem. Subnetting involves the use of an IP address and subnet mask to determine whether a destination network is local or remote. I'm going to explain the basics of IP addressing, reasons to subnet, and the logic behind slicing up an IP network. IP addressing
IP addresses are perhaps the most fundamental components of modern networking. They are the identifiers that computers use to talk to each other on a TCP/IP-based network (including the Internet, most corporate networks, and many home networks). An IP address is a 32-bit number represented in a dotted decimal format. This address is further divided into four sections, with each section representing eight bits or an "octet". So this 32-bit, four-octet number is in many ways similar to a telephone number. Like a telephone number, it must be unique. Although there are roughly four billion or so available IP addresses, we still need to be picky with how many are given out. With the dawn of the Internet and our increasingly IP-enabled world, every address counts. In the US, the American Registry for Internet Numbers (ARIN) handles the management of public IP addresses. Typically, addresses are handed out by ISPs and fall under their management. An ISP will have multiple pools of addresses (and subnets) to assign. Also, as defined in RFC 1918, private networks are available. These are for internal use and are not routed through the Internet. Private addresses look like this: 10.0.0.0 -- 10.255.255.255 172.16.0.0 -- 172.31.255.255 192.168.0.0 -- 192.168.255.255 In IP version 4 (IPv4), the standard that currently dominates the networks of the world, IP addresses fall into one of five classes: A, B, C, D, or E. Classes D and E are for multicast and experimental uses, respectively, and are less common. A, B, and C networks are defined in this table:

First Octet	Class	Subnet mask
1-127	Class A	255.0.0.0
128-191	Class B	255.255.0.0
192-223	Class C	255.255.255.0

This use of classes represents the original method of dividing networks, but it is somewhat dated. There just isn't enough middle ground between B and C. While a standard class C network contains 254 addresses (too few addresses for a moderate-size company), a class B network has 65,534 (far too many for the average network). This is where subnetting comes into play.