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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)
                9  Internet Protocol Version 4 (IP, IPv4)
                     9  IP Addressing
                          9  IP Addressing Concepts and Issues

Previous Topic/Section
IP Address Size, Address Space and "Dotted Decimal" Notation
Previous Page
Pages in Current Topic/Section
12
3
Next Page
IP Addressing Categories (Classful, Subnetted and Classless) and IP Address Adjuncts (Subnet Mask and Default Gateway)
Next Topic/Section

IP Basic Address Structure and Main Components: Network ID and Host ID
(Page 3 of 3)

Location of the Division Between Network ID and Host ID

One difference between IP addresses and phone numbers is that the dividing point between the bits used to identify the network and those that identify the host isn't fixed. It depends on the nature of the address, the type of addressing being used, and other factors. Let's take the example from the last topic, 227.82.157.177. It is possible to divide this into a network identifier of “227.82” and a host identifier of “157.177”. Alternately, the network identifier might be “227” and the host identifier “82.157.177” within that network.

To express the network and host identifiers as 32-bit addresses, we add zeroes to replace the missing “pieces”. In the latter example just above, the address of the network becomes “227.0.0.0” and the address of the host “0.82.157.177”. (In practice, network addresses of this sort are routinely seen with the added zeroes; network IDs are not as often seen in 32-bit form this way.)

Lest you think from these examples that the division must always be between whole octets of the address, it's also possible to divide it in the middle of an octet. For example, we could split the IP address 227.82.157.177 so there were 20 bits for the network ID and 12 bits for the host ID. The process is the same, but determining the dotted decimal ID values is more tricky because here, the “157” is “split” into two binary numbers. The results are “227.82.144.0” for the network ID and “0.0.0.13.177” for the host ID, as shown in Figure 58.


Figure 58: Mid-Octet IP Address Division

Since IP addresses are normally expressed as four dotted-decimal numbers, educational resources often show the division between the Network ID and Host ID occurring on an octet boundary. However, it’s essential to remember that the dividing point often appears in the middle of one of these eight-bit numbers. In this example, the Network ID is 20 bits long and the Host ID 12 bits long. This results in the third number of the original IP address, 157, being split into 144 and 13.

 


The place where the “line is drawn” between the network ID and the host ID must be known in order for devices such as routers to know how to interpret the address. This information is conveyed either implicitly or explicitly depending on the type of IP addressing in use. I describe this in the following topic.

Key Concept: The basic structure of an IP address consists of two components: the network ID and host ID. The dividing point of the 32-bit address is not fixed, but rather, depends on a number of factors, and can occur in a variety of places, including in the middle of a dotted-decimal octet.



Previous Topic/Section
IP Address Size, Address Space and "Dotted Decimal" Notation
Previous Page
Pages in Current Topic/Section
12
3
Next Page
IP Addressing Categories (Classful, Subnetted and Classless) and IP Address Adjuncts (Subnet Mask and Default Gateway)
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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