Illustrated TCP/IP Illustrated TCP/IP
by Matthew G. Naugle
Wiley Computer Publishing, John Wiley & Sons, Inc.
ISBN: 0471196568   Pub Date: 11/01/98  

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Chapter 37
The IP Address Scheme

Every systems engineer who understands IP, understands the IP address scheme. It can be the most confusing aspect of IP, however, it must be learned. Do not confuse this addressing structure with that of media (Ethernet) address. The ideas and concepts that evolved the protocol of TCP/IP were devised separate from any datalink protocols of Ethernet and Token Ring. Hosts were not attached to a local high–speed network (like Ethernet or Token Ring). Hosts communicated with each other through low–speed, point–to–point serial lines (telephone lines). Therefore, an addressing scheme to identify TCP/IP hosts and where they were located was implemented. The addressing scheme used to identify these hosts is called the 32–bit IP address. This is also known as a protocol address.

There are two types of network addressing schemes used with IP:

Classless. The full address range can be used without regard to bit reservation for classes. This type of addressing scheme is primarily not used in direct host assignment. The scheme is directly applied to the routing tables of the Internet and ISPs.
Classful. The original (RFC 791) segmentation of the 32–bit address into specific classes denoting networks and hosts.

The fun part is that the range of addresses (32 bits for IPv4) available are used for both classless and classful addressing. Most of us will never have to worry about the classless range of IP addressing, for it is used on the Internet itself and not on customer networks. It provides an easy method with which to reduce the routing tables and allow large address ranges to be provided to the ISPs. The first part of this section will deal with classful, since it started first and is continuing to be used on many networks. It is confusing, but keep reading.

The IP Address Scheme

•  Two types of addressing schemes for IPv4:
•  Classful (based on RFC 791)—The original style of addressing based on the first few bits of the address
•  Generally used in customer sites
•  Classless—The new style of addressing that disregards the Class bits of an address and applies a variable 32 prefix (mask) to determine the network number
•  Generally used by the global routing tables and ISPs
•  Enables very efficient routing, smaller routing tables
•  Enables efficient IP address allocation (to the ISPs) and assignment (to the ISP customer)

The second part of this section will deal with classless addressing and the concepts of CIDR (Classless InterDomain Routing), Variable Length Subnet Masks (VLSM), and supernetting.


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