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 58
Classful IP Address Review

Let’s review. All IPv4 addresses are 32 bits in length and are the grouping of 4 bytes that represents both a network number and host number. This number is usually represented in decimal. With the first bit reserved (set to 0xxxxxxx) in a Class A address, the network numbers can range from 1 to 126. Number 127 is reserved as a local loopback IP address and must not be assigned to a network number and transmitted onto the network. With the first 2 bits reserved in a Class B (10xxxxxx) or 3 bits in a Class C (110xxxxx) address, the network numbers for Class B range from 128.1.0.0 to 191.255.0.0, and for Class C they range from 192.1.1.0 to 223.255.255.0.

Examples:

192.1.1.1 Node assigned with a host ID of 1, located on a Class C network of network 192.1.1.0
200.6.5.4 Node assigned with a host ID of 4, located on a Class C network of 200.6.5.0
150.150.5.6 Node assigned with a host ID of 5.6, located on a Class B network of 150.150.0.0
9.6.7.8 Node assigned with a host ID of 6.7.8, located on a Class A network of 9.0.0.0
128.1.0.1 Node assigned with a host ID of 0.1, located on a Class B network of 128.1.0.0

Notice that to represent a network number only, only the network number is written. The host field will be set to 0. This type of network number display will become apparent when looking at routing tables.

Classful IP Address Review

•  In the first field:
•  Class A has the range of 1–126
•  Class B has the range of 128–191
•  Class C has the range of 192–223
•  Class D has the range of 224–239
•  Subnetting is the ability to place a mask over the host portion of the address to yield subnets.
•  Allows for another level of hierarchy; efficient for routing
•  RIP version 1 has problems with variable subnet masks.

For those not familiar with binary, you need to memorize the starting and stopping points of the first byte of an IP address:

Class A 1–126 in the first field
Class B 128–191 in the first field
Class C 192–223 in the first field

Subnetting is the ability to move a mask over the bits normally associated with a host address and reclaim these bits as a subnet number. The mask can use 22 bits for a Class A address, 14 bits for a Class B address, and 6 bits for a Class C address.


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