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OSI Reference Model

Introduction to TCP/IP

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Understanding Switching

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What Is Layer 3 Switching?

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Understanding Quality of Service

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Understanding Virtual Private Networks (VPN)

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Network Management Basics

The Internet




Introduction to TCP/IP

IPv4 Addressing

Most IP addresses today use IP version 4—we’ll talk about IP version 6 later. IPv4 addresses are 32 bits long and are usually written in “dot” notation. An example would be The Internet is actually a lot of small local networks connected together. Part of an IP address identifies which local network, and part of an IP address identifies a specific system or host on that local network. What part of an IP address is for the “network” and what part is for the “host” is determined by the class or the subnet.

IP Addressing—Three Classes


Before the introduction of subnet masks, the only way to tell the network part of an IP address from the host part was by its class.
Class A addresses have 8 bits (one octet) for the network part and 24 bits for the host part. This allows for a small number of large networks.
Class B addresses have 16 bits each for the network and host parts.
Class C addresses have 24 bits for the network and 8 bits for the host. This allows for a fairly large number of networks with up to 254 systems on each.

To summarize:

IPv4 addresses are 32 bits with a network part and a host part.
Unless you are using subnets, you divide an IP address into the network and host parts based on the address class.
The network part of an address is used for routing packets over the Internet. The host part is used for final delivery on the local net.

IP Addressing—Class A

Here’s an example of a class A address. Any IPv4 address in which the first octet is less than 128 is by definition a class A address.
This address is for host #222.135.17 on network #10, although the host is always referred to by its full address.


 - Network # 10
 - Host # 222.135.17
 - Range of class A network IDs: 1–126
 - Number of available hosts: 16,777,214

IP Addressing—Class B

Here’s an example of a class B address. Any IPv4 address in which the first octet is between 128 and 191 is by definition a class B address


 - Network # 128.128
 - Host # 141.245
 - Range of class B network IDs: 128.1–191.254
 - Number of available hosts: 65,534

IP Addressing—Class C

Here’s an example of a class C address. Most IPv4 addresses in which the first octet is 192 or higher are class C addresses, but some of the higher ranges are reserved for multicast applications.


 -Network # 192.150.12
 -Host # 1
 -Range of class C network IDs: 192.0.1–223.255.254
 -Number of available hosts: 254

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