Understanding Subnets

Understanding Subnets

 The IP addressing scheme provides a flexible solution to the task of addressing thousands of networks, but it is not without problems. The original designers did not envision the Internet growing
as large as it has; at that time, a 32-bit address seemed so large that they quickly divided it into different classes of networks to facilitate routing rather than reserving more bits to manage the
growth in network addresses. To solve this problem, and to create a large number of new network addresses, another way of dividing the 32-bit address was developed, called subnetting. An IP subnet modifies the IP address by using host ID bits as additional network address bits. In other words, the dividing line between the network address and the host ID is moved to the right, thus creating additional networks but reducing the number of hosts that can belong to each network. When IP networks are subnetted, they can be routed independently, which allows a much better use of address space and available bandwidth. To subnet an IP network, you define a bit mask, known as a subnet mask, in which a bit pattern of consecutive 1s followed by consecutive 0s is ANDed with the IP address to produce a network address with all 0s in the host ID.
Working out subnet masks is one of the most complex tasks in network administration and is not for the faint of heart. If your network consists of a single segment (in other words, there
are no routers on your network), you will not have to use this type of subnetting, but if you have two or more segments (or subnets), you will have to make some sort of provision for distributing
IP addresses appropriately. Using a subnet mask is the way to do just that. The subnet mask is similar in structure to an IP address in that it has four parts, or octets, but it works a bit like a template that, when superimposed on top of the IP address, indicates
which bits in the IP address identify the network and which bits identify the host. In binary, if a bit is on (set to 1) in the mask, the corresponding bit in the address is interpreted as a network
bit. If a bit is off (reset to 0) in the mask, the corresponding bit in the address is part of the host ID. The 32-bit value may then be converted to dotted decimal notation for human consumption.
Sometimes, you will use only one subnet mask to subnet your network. Variable Length Subnet Masking (VLSM) is the practice of using more appropriate varied subnet masks with the same
classful network for the different subnet sizes. A classful network is one subnetted to the default boundaries of network and host bits, based on the class of IP address. A subnet is only known and understood locally; to the rest of the Internet, the address is still
interpreted as a classful IP address (and maybe even as a group of classful addresses) if an entity has administrative control over a contiguous block of such addresses. Table 3.2 shows how this
works for the standard IP address classes. Routers then use the subnet mask to extract the network portion of the address so that they can compare the computed network address with the routing table entry corresponding to the mask used and send the data packets along the proper route on the network.

TABLE 3 . 2 Default Subnet Masks for Standard IP Address Classes

Because pretty much all the Class A and Class B networks are taken, you are most likely to encounter subnet-related issues when working with a Class C network or with any private address space. In the next section, you’ll get a detailed look at how to subnet a Class C network.

Why Subnet?

When faced with the choice of whether or not to subnet your network, you must remember several of the advantages to subnetting. The following list summarizes the advantages of the subnetting solution:

• It minimizes network traffic, decreasing congestion.
• It isolates networks from others.
• It increases performance.
• It optimizes use of IP address space.
• It enhances the ability to secure a network.