The Layer 3 address convention in the TCP/IP world uses a 32 bit binary number to logically identify each node on the network. The communicating nodes are referred to as hosts and the 32 bit binary address number is the IP Address. This 32 bit number is represented by breaking the 32 bits into four groups of eight bits each and representing each eight bit byte with the decimal value equivalent of the binary number.
This is referred to as the Dotted-Decimal Notation or as an Octet String. For example, assume a station is assigned the following address:
To represent this address in Dotted-Decimal Notation, the address is first broken up into four bytes, as follows:
10000010 00000100 00101100 00000001
Next, each byte is converted from binary to decimal, as follows:
10000010 = 130
00000100 = 4
00101100 = 44
00000001 = 1
The result is written in Dotted-Decimal Notation as:
Although the representation of the address consists of four decimal numbers separated by dots, the underlying meaning of the address can only be understood by evaluating the 32-bit binary number. This 32-bit number is used to identify each communicating device on the network.
In general there are two fundamental design relationships that can be identified in the construction of a network infrastructure. We can call these flat networks versus hierarchical networks. In a flat network every device is directly reachable by every other device. In a hierarchical network the world is divided into separate locations and devices are assigned to a specific location. The advantage of the hierarchical design is that the devices that interconnect the parts of the communications infrastructure need only know how to reach the intended destination location without having to keep track of the individual devices at each location. This device is a router. It makes a forwarding decision by looking at that part of the station address that identifies the location where the station resides.
All addressing in hierarchical networks may be considered to have two distinct parts. We might refer to these two parts as a locator portion and a node portion.
The locator portion identifies the location at which the node resides. This "locator & node" address operates at Layer 3 of the OSI model, the Network Layer.
Different vendors use different terms to refer to the locator and node portions of the address. For example, Novell NetWare calls them the "network" and "node"
number. DECNet refers to the "area" and "node" number. TCP/IP uses two different terms to refer to the locator portion of the IP address. In a very fundamental
sense these two terms are synonymous. They are the "network" and the "subnetwork". While it is true that the concept being expressed in IP routing is that of a
"network" which is sub-divided into smaller locations referred to as "subnetworks" it must be remembered that a "subnetwork" may be further subdivided into
smaller locations which would each also be referred to as a "subnetwork". The difference is mainly semantic in nature. Which of the following descriptions would
you like to adopt?
In the IP world there is a configuration parameter that defines which bits in a Layer 3 address are used to differentiate between the locator portion and the node portion. This parameter is called the "address mask" (also called the "Subnet Mask"). We'll talk in-depth about the address mask under the SUBNET MASK topic (this is the Next Topic), but the simple explanation is that for each '1' bit in the mask, the corresponding bit in the address is considered to be part of the locator portion. Whether you refer to this as a subnet or a "Class A" or "Class B" network; it makes no difference. (The ADDRESS CLASSES topic describes network classes.) To understand IP addressing you must understand this fundamental rule, again:
For each '1' bit in the mask, the corresponding bit in the address is considered to be part of the locator portion of the address.
The remaining bits are considered to identify a specific node at that location.
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