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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
1
23
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 1 of 3)

As I mentioned in the IP addressing overview, one of the ways that IP addresses are used is to facilitate the routing of datagrams in an IP internet. This is made possible because of the way that IP addresses are structured, and how that structure is interpreted by network routers.

Internet IP Address Structure

As we just saw, each version 4 IP address is 32 bits long. When we refer to the IP address we use a dotted-decimal notation, while the computer converts this into binary. However, even though these sets of 32 bits are considered a single “entity”, they have an internal structure containing two components:

  • Network Identifier (Network ID): A certain number of bits, starting from the left-most bit, is used to identify the network where the host or other network interface is located. This is also sometimes called the network prefix or even just the prefix.

  • Host Identifier (Host ID): The remainder of the bits are used to identify the host on the network.

Note: By convention, IP devices are often called hosts for simplicity, as I do throughout this Guide. Even though each host usually has a single IP address, remember that IP addresses are strictly associated with network-layer network interfaces, not physical devices, and a device may therefore have more than one IP address.


As you can see in Figure 57, this really is a fairly simple concept; it's the same idea as the structure used for phone numbers in North America. The telephone number (401) 555-7777 is a ten-digit number usually referred to as a single “phone number”. However, it has a structure. In particular, it has an area code (“401”) and a local number (“555-7777”).


Figure 57: Basic IP Address Division: Network ID and Host ID

The fundamental division of the bits of an IP address is into a network ID and host ID. Here, the network ID is 8 bits long, shown in cyan, and the host ID is 24 bits in length.

 


Previous Topic/Section
IP Address Size, Address Space and "Dotted Decimal" Notation
Previous Page
Pages in Current Topic/Section
1
23
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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