The Hierarchical IP Addressing Scheme

An IP address is made up of 32 bits of information. These bits are divided into four sections (sometimes called octets or quads) containing 1 byte (8 bits) each. There are three common methods for specifying an IP address:

■ Binary, as in 10000010.00111001.00011110.00111000

All of these examples represent the same IP address.

The 32-bit IP address is a structured address, or hierarchical address, as opposed to a flat address, or nonhierarchical one. Although IP could have used either flat or hierarchical addressing, its designers chose hierarchical addressing—for a very good reason, as you will see.

What's the difference between these two types of addressing? A good example of a flat addressing scheme is a driver's license number. There's no partitioning to it; the range of legal numbers isn't broken up in any meaningful way (say, by county of residence or date of issuance). If this method had been used for IP addressing, every machine on the Internet would have needed a totally unique address, just as each driver's license number is unique. The good news about flat addressing is that it can handle a large number of addresses, namely 4.3 billion (a 32-bit address space with two possible values for each position—either 0 [zero] or 1 [one]— giving you 232, which equals approximately 4.3 billion). The bad news—and the reason why flat addressing isn't used in IP—relates to routing. If every address were totally unique, every router on the Internet would need to store the address of each and every other machine on the Internet. It would be fair to say that this would make efficient routing impossible, even if only a fraction of the possible addresses were used.

The solution to this dilemma is to use a hierarchical addressing scheme that breaks the address space up into ordered chunks. Telephone numbers are a great example of this type of addressing. The first section of a telephone number, the area code, designates a very large area; the area code is followed by the prefix, which narrows the scope to a local calling area. The final segment, the customer number, zooms in on the specific connection. By looking at a number like 256-233-xxxx, you can quickly determine that the number is located in the northern part of Alabama (area code 256) in the Athens/East Limestone area (the 233 exchange).

IP addressing works the same way. Instead of treating the entire 32 bits as a unique identifier, one part of the IP address is designated as the network address (or network ID) and the other part as a node address (or host ID), giving it a layered, hierarchical structure.

The network address uniquely identifies each network. Every machine on the same network shares that network address as part of its IP address, just as the address of every house on a street shares the same street name. In the IP address, for example, 130.57 is the network address.

The node address is assigned to, and uniquely identifies, each machine in a network, just as each house on the same street has a different house number. This part of the address must be unique because it identifies a particular machine—an individual, as opposed to a network. This number can also be referred to as a host address. In the sample IP address, .30.56 is the node address. Together as the IP address, the network address and the node address uniquely identify a device within an internetwork.

The designers of the Internet decided to create classes of networks based on network size. For the small number of networks possessing a very large number of nodes, they created the Class A network. At the other extreme is the Class C network, reserved for the numerous networks with a small number of nodes. The class distinction for networks in between very large and very small is predictably called a Class B network. The default subdivision of an IP address into a network and node address is determined by the class designation of your network. However, Classless Inter-Domain Routing (CIDR) has effectively done away with these classful designations. You will still hear and should still know the meaning behind the classful designations of addresses because they are important to understanding IP addressing. However, when working with IP addressing in practice, CIDR is more important to know. Table 1.2 provides a summary of the three classes of networks, which will be described in more detail in the following sections.

TABLE 1.2 Network Address Classes


Mask Bits

Leading Bit Pattern

Decimal Range of First Octet of IP Address

Assignable Networks

Maximum Nodes per Network



















To ensure efficient routing, Internet designers defined a mandate for the leading bits section of the address for each different network class. For example, because a router knows that a Class A network address always starts with a 0, the router can quickly apply the default mask, if necessary, after reading only the first bit of the address. Table 1.2 illustrates how the leading bits of a network address are defined. When considering the subnet masking between network and host addresses, the number of bits to mask is important. For example, in a Class A network, 8 bits are masked, making the default subnet mask, while in a Class C, 24 bits are masked, making the default subnet mask

Some IP addresses are reserved for special purposes and shouldn't be assigned to nodes by network administrators. Table 1.3 lists some of the reserved IP addresses. See RFC 3330 for others.

TABLE 1.3 Special Network Addresses



Entire IP address set to all 0s

Depending on the mask, this network (i.e., the network or subnet that you are currently a part of) or this host on this network

A routing table entry of all 0s with a mask of all 0s

Used as the default gateway entry. Any destination address masked by all 0s produces a match for the all 0s reference address. Because the mask has no 1s, this is the least desirable entry but will be used when no other match exists.

Network address 127

Reserved for loopback tests. Designates the local node and allows that node to send a test packet to itself without generating network traffic.

Node address of all 0s

Used when referencing a network without referring to any specific nodes on that network. Usually used in routing tables.

Node address of all 1s

Broadcast address for all nodes on the specified network, also known as a directed broadcast; for example, means all nodes on the Class B network 128.2. Routing this broadcast is configurable on certain routers. with a mask of

The "link local" block used for auto-configuration and communication between devices on a single link. Communication cannot occur across routers. Microsoft uses this block for Automatic Private IP Addressing (APIPA).

Entire IP address set to all 1s (same as

Broadcast to all nodes on the current network; sometimes called a limited broadcast or an "all 1s broadcast." This broadcast is not routable.

The private-use blocks for Classes A, B, and C. As noted in RFC 1918, the addresses in these blocks must never be allowed into the Internet, making them acceptable for simultaneous use behind NAT servers and non-Internet-connected internetworks.

In the following sections, we will look at the three different network types.

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  • william stovall
    Why is it called hierarchical ip addressing scheme?
    3 years ago
  • teuvo
    How is this hierarchical addressing achieved?
    1 year ago
  • Melissa Brown
    How to use a hierarchical internet protocol network address
    1 year ago
  • gerardo
    How to assign hierarchial ip addressin a network?
    1 year ago
  • faramir
    What is ip address hierarchical allocation?
    1 year ago
    Why internet protocol address is arranged hierarchical?
    1 year ago
  • Judith
    Why internet protocol address are organized hierarchical?
    1 year ago
  • noble
    How is this hierarchical addressing achieved Explain.?
    1 year ago
  • Jamie
    Why are ip addresses hierarchical?
    1 year ago
  • Zack
    How the hierarchical address is used in IP routing?
    10 months ago
  • Charley
    Does the loT devices include an address how is this hierarchical addressing achieved?
    9 months ago

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