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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 Basic Address Structure and Main Components: Network ID and Host ID
Previous Page
Pages in Current Topic/Section
Next Page
Number of IP Addresses and Multihoming
Next Topic/Section

IP Addressing Categories (Classful, Subnetted and Classless) and IP Address Adjuncts (Subnet Mask and Default Gateway)
(Page 2 of 2)

Classless Addressing

In the classless system, the classes of the original IP addressing scheme are tossed out the window. The division between the network ID and host ID can occur at an arbitrary point, not just on octet boundaries like in the “classful” scheme.

The dividing point is indicated by putting the number of bits used for the network ID, called the prefix length, after the address (recall that the network ID bits are also sometimes called the network prefix, so the network ID size is the prefix length). For example, if is part of a network where the first 27 bits are used for the network ID, that network would be specified as The “/27” is conceptually the same as the subnet mask, since it has 27 one bits followed by 5 zeroes.

Key Concept: An essential factor in determining how an IP address is interpreted is the addressing scheme in which it is used. The three methods, arranged in increasing order of age, complexity and flexibility, are “classful” addressing, subnetted “classful” addressing, and classless addressing.

Did I just confuse the heck out of you? Sorry—and don't worry. I'm simply introducing the concepts of “classful”, subnetted and classless addressing and showing you how they impact the way the IP address is interpreted. This means of necessity that I have greatly summarized important concepts here. All three methods are explained in their own sections in full detail.

IP Address Adjuncts: Subnet Mask and Default Gateway

As you can see, in the original “classful” scheme the division between network ID and host ID is implied. However, if either subnetting or classless addressing is used, then the subnet mask or “slash number” are required to fully qualify the address. These numbers are considered adjuncts to the IP address and usually mentioned “in the same breath” as the address itself, because without them, it is not possible to know where the network ID ends and the host ID begins.

One other number that is often specified along with the IP address for a device is the default gateway identifier. In simplest terms, this is the IP address of the router that provides default routing functions for a particular device. When a device on an IP network wants to send a datagram to a device it can't see on its local IP network, it sends it to the default gateway which takes care of routing functions. Without this, each IP device would also have to have knowledge of routing functions and routes, which would be inefficient. See the sections on routing concepts and TCP/IP routing protocols for more information.

Previous Topic/Section
IP Basic Address Structure and Main Components: Network ID and Host ID
Previous Page
Pages in Current Topic/Section
Next Page
Number of IP Addresses and Multihoming
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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