Patrick Stoddard
NetworkNewz Editor

Each physical section of a network is usually assigned a unique block of IP addresses. This is done by assigning the segment a network address and subnet mask. Every interface connected to that network segment is then assigned an address from within the block of addresses and it is also configured using the subnet mask. Sometimes multiple blocks of IP addresses will co-exist on a single network segment, utilizing the same physical cabling network but operating as separate logical network segments.

When designing and configuring a subnet, a network address and subnet mask is defined for each block of IP addresses. The network address defines the start of the IP address block and the subnet mask determines how large the block is. The subnet mask also determines which portion of the IP address is used to identify the network and which part of the IP address is used to identify the host or interface.

To properly understand IP addresses and subnet masks, you should think of them binary numbers. Each IP address and each subnet can be written as four groups of eight binary digits (ones and zeroes), separated by periods (called dots). In the subnet mask, when represented in binary notation, the 1 bits identify the network and the 0 bits identify the interface address.

TCP/IP software uses a logical "AND" operation to find out what subnet an address belongs to. When a 0 is ANDed with either a 0 or a 1, the result is a 0. When a 1 is ANDed with a 1, the result is 1:

0 AND 0 = 0
0 AND 1 = 0
1 AND 0 = 0
1 AND 1 = 1

Classless or Classful Subnetting
When TCP/IP was originally developed, there were only a limited number of computer systems on the Internet and IP addresses were plentiful. The methods for distributing IP addresses assumed that there would always be enough addresses to go around. The available address space was broken up into arbitrary networks of three different sizes: Class A, Class B, and Class C. Class C networks were for networks with fewer than 255 hosts. Class B networks were for networks with fewer than 65,535 hosts, and Class A network were for networks with more than 65,535 or more hosts. This is commonly referred to as Classless networking.

The Classless rules for networking and subnetting tend to be very lavish with IP addresses, frequently wasting large numbers of addresses. As the Internet began to grow, the Classless rules of routing were replaced with a newer set of rules called Classful Inter -Domain Routing (CIDR). The Classful rules make much better use of available IP addresses but unfortunately there are still some pieces of equipment and software applications that were written using the original Classless IP rules. If your network has only equipment and software that is Classful compliant, then you should use Classful rules for subnetting. If you have older equipment or software, particularly older Ethernet-TCP/IP printer cards or older UNIX systems, then you should use Classless subnetting rules.

A common situation encountered on TCP/IP networks is the need to break a single Class C network into two subnets of IP. Using Classful IP rules the subnet mask most commonly used to do this is 255.255.255.192, which results in 2 subnets with 62 usable addresses in each.

Now I know you are wondering, if we break a group of 254 addresses into two pieces, why do we only end up with 2 groups of 62? Where did all of the other addresses go? Under Classless and Classful IP rules, within a subnet, the addresses with a host portion that is all zeroes or all ones have special meanings. The host address with all ones is the broadcast address for the subnet. Packets addressed to the broadcast address are received by all devices on the network. The address with a host portion that is all zeroes is the same as the network address and cannot be used as an actual host address because this causes confusion with certain network commands and messages. This is why in a Class C network, with numbers 0 through 255 available, you lose 0 (binary 00000000) and 255 (binary 11111111) and can use only 254 addresses (1 thru 254).