Internet Addressing
In a Nutshell - CIW Course Section 1, Part C, Chapter 5
Internet Addressing
To ensure that each host on the Internet has a unique IP address, a central authority called the International Corporation of Assigned Names and Numbers (ICANN) issues all Internet addresses.
Most Internet addresses contain a network portion and a host portion. The network portion precedes the host portion. Internet addresses are specified as four fields seperated by full stops. These are typically written in dotted decimal notation. Each field has a value from 0 to 255.208.156.21.110
In this example the network portion is 208.158.21 and the host portion is 110. The network portion is determined by the class of the address.
Internet Address Classes
Internet addresses are divided into five classes: A, B, C, D and E. These classes provide structure to the address range and are shown below:
IP Addressing Rules
Internet addresses must follow several guideline to function correctly. Not all addresses within the above ranges may be used.
Loopback Address
The loopback address 127 cannot be used as an internet address. This address allows a client and server on the same host to communicate with each other. This is ideal for testing and troubleshooting. This loopback address is often assigned the name localhost.
Broadcast Address
The broadcast address is 255 and is used to send messages to all network hosts. The address 255.255.255.255 is known as a limited broadcadt and is used for configuration at boot up. In the earlier IP example 208.156.21.255 will broadcast to all hosts on the local network.
Reserved IP Addressing
ICANN has reserved three blocks of IP addresses for private networks:
- 10.0.0.0 to 10.255.255.255
- 172.16.0.0 to 172.31.255.255
- 192.168.0.0 to 192.168.255.255
These are normally used for company networks where any access to the Internet is via a mediating gateway.
Subnet Masks
A subnet mask is a 32-bit number (like an IP address) with a one-to-one correlation to each bit of the IP address.
The subnet mask server two main purposes:
- Distinguish the network and host portions of an IP address
- Specify whether a destination address is local or remote
The following shows the default subnet masks for the first three classes:
| 255.0.0.0 | Class A |
| 255.255.0.0 | Class B |
| 255.255.255.0 | Class C |
Class D and E do not have hosts, and therefore do not need subnet masks