Class A Networks- Understanding IP

In a Class A network, the first byte is the network address, and the remaining 3 bytes are used for the node addresses. The Class A format is Network.Node.Node.Node.

For example, in the IP address 49.22.102.70, 49 is the network address, and 22.102.70 is the node address. Every machine on this particular network would have the distinctive

Understanding IP Addressing

network address of 49. Within that network, however, you could have a large number of machines.

There are 126 possible Class A network addresses. Why? The length of a Class A network address is 1 byte, and the first bit of that byte is reserved, so 7 bits in the first byte remain available for manipulation. This means that the maximum number of Class A networks is 128. (Each of the 7 bit positions that can be manipulated can be either a 0 or a 1, and this gives you a total of 27 positions, or 128.) But to complicate things further, it was also decided that the network address of all 0s (0000 0000) would be reserved. This means that the actual number of usable Class A network addresses is 128 minus 1, or 127. Also, 127 is a reserved number (a network address of 0 followed by all 1s [0111 1111]), so you actually start with 128 addresses minus the 2 reserved, and you’re left with 126 possible Class A network addresses.

Each Class A network has 3 bytes (24 bit positions) for the node address of a machine, which means that there are 224, or 16,777,216, unique combinations. Because addresses with the two patterns of all 0s and all 1s in the node bits are reserved, the actual maximum usable number of nodes for a Class A network is 224 minus 2, which equals 16,777,214.

Class B Networks

In a Class B network, the first 2 bytes are assigned to the network address, and the remaining 2 bytes are used for node addresses. The format is Network.Network.Node.Node.

For example, in the IP address 130.57.30.56, the network address is 130.57, and the node address is 30.56.

The network address is 2 bytes, so there would be 216 unique combinations. But the Internet designers decided that all Class B networks should start with the binary digits 10. This leaves 14 bit positions to manipulate; therefore, there are 16,384 (or 214) unique Class B networks.

This gives you an easy way to recognize Class B addresses. If the first 2 bits of the first byte can be only 10, that gives you a decimal range from 128 up to 191 in the first octet of the IP address. Remember that you can always easily recognize a Class B network by looking at its first byte, even though there are 16,384 different Class B networks. If the first octet in the address falls between 128 and 191, it is a Class B network, regardless of the value of the second octet.

A Class B network has 2 bytes to use for node addresses. This is 216 minus the two patterns in the reserved- exclusive club (all 0s and all 1s in the node bits) for a total of 65,534 possible node addresses for each Class B network.

Class C Networks

The first 3 bytes of a Class C network are dedicated to the network portion of the address, with only 1 byte remaining for the node address. The format is Network.Network.Network.Node.

In the example IP address 198.21.74.102, the network address is 198.21.74, and the node address is 102.

In a Class C network, the first three bit positions are always binary 110. Three bytes, or 24 bits, minus 3 reserved positions leaves 21 positions. There are therefore 221 (or 2,097,152) possible Class C networks.

The lead bit pattern of 110 equates to decimal 192 and runs through 223. Remembering our handy easy- recognition method, this means you can always spot a Class C address if the first byte is in the range 192–223, regardless of the values of the second and third bytes of the IP address.

Each unique Class C network has 1 byte to use for node addresses. This leads to 28, or 256, minus the two special patterns of all 0s and all 1s, for a total of 254 node addresses for each Class C network.

Class D networks, used for multicasting only, use the address range 224.0.0.0 to 239.255.255.255 and are used, as in broadcasting, as destination addresses only. Class E networks (reserved for future use at this point) cover 240.0.0.0 to 255.255.255.255. Addresses in the Class E range are considered within the experimental range.

Subnetting a Network

If an organization is large and has lots of computers or if its computers are geographically dispersed, it makes good sense to divide its colossal network into smaller ones connected by routers. These smaller networks are called subnets. The benefits of using subnets are as follows:

Reduced Network Traffic We all appreciate less traffic of any kind, and so do networks. Without routers, packet traffic could choke the entire network. Most traffic will stay on the local network— only packets destined for other networks will pass through the router and to another subnet. This traffic reduction also improves overall performance.

Simplified Management It’s easier to identify and isolate network problems in a group of smaller networks connected together than within one gigantic one.

Understanding the Benefits of Subnetting
To understand one benefit of subnetting, consider a hotel or office building. Say that a hotel has 1,000 rooms with 75 rooms to a floor. You could start at the first room on the first floor and number it 1; then when you get to the first room on the second floor, you could number it 76 and keep going until you reach room 1,000. But someone looking for room 521 would have to guess on which floor that room is located. If you were to “subnet” the hotel, you would identify the first room on the first floor with the number 101 (1 = Floor 1 and 01 = Room 1), the first room on the second floor with 201, and so on. The guest looking for room 521 would go to the fifth floor and look for room 21.

An organization with a single network address (comparable to the hotel building mentioned in the sidebar “Understanding the Benefits of Subnetting”) can have a subnet address for each individual physical network (comparable to a floor in the hotel building). Each subnet is still part of the shared network address, but it also has an additional identifier denoting its individual subnetwork number. This identifier is called a subnet address.

Subnetting solves several addressing problems:

       If an organization has several physical networks but only one IP network address, it can handle the situation by creating subnets.

       Because subnetting allows many physical networks to be grouped together, fewer entries in a routing table are required, notably reducing network overhead.

           These things combine collectively to yield greatly enhanced network efficiency.

The original designers of the Internet Protocol envisioned a small Internet with only tens of networks and hundreds of hosts. Their addressing scheme used a network address for each physical network. As you can imagine, this scheme and the unforeseen growth of the Internet created a few problems. The following are two examples:

Not Enough Addresses A single network address can be used to refer to multiple physical networks, but an organization can request individual network addresses for each one of its physical networks. If all of these requests were granted, there wouldn’t be enough addresses to go around.

Gigantic Routing Tables If each router on the Internet needed to know about every physical network, routing tables would be impossibly huge. There would be an overwhelming amount of administrative overhead to maintain those tables, and the resulting physical overhead on the routers would be massive (CPU cycles, memory, disk space, and so on). Because routers exchange routing information with each other, an additional, related consequence is that a terrific overabundance of network traffic would result.

Although there’s more than one way to approach these problems, the principal solution is the one that I’ll cover in this book— subnetting. As you might guess, subnetting is the process of carving a single IP network into smaller logical subnetworks. This trick is achieved by subdividing the host portion of an IP address to create a subnet address. The actual subdivision is accomplished through the use of a subnet mask (covered later in the chapter).

In the following sections, you will see exactly how to calculate and apply subnetting.

Leave a Reply

Your email address will not be published. Required fields are marked *