By Ralph Becker
Updated September 7, 1999
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More Restrictive Subnet Masks
CIDR -- Classless InterDomain Routing
Allowed Class A Subnet and Host IP addresses
Allowed Class B Subnet and Host IP addresses
Allowed Class C Subnet and Host IP addresses
References and Sources on the Internet
This talk will cover the basics of IP addressing and subnetting. Topics covered will include:
· Whatis an IP Address?
· What are Classes?
· What is a Network Address?
· What are Subnet Masks and Subnet Addresses?
· How are Subnet Masks defined and used?
· How can all this be applied?
· What is CIDR?
An IP (Internet Protocol) address is a unique identifier for a node or host connection on an IP network. An
IP address is a 32 bit binary number usually represented as 4decimal values, each representing 8 bits, in
the range 0 to 255 (known as octets) separated by decimal points. This is known as "dotted decimal"
It is sometimes useful to view the values in their binary form.
140 .179 .220 .200
Every IP address consists of two parts, one identifying the network and one identifying the node.The
Class of the address and the subnet mask determine which part belongs to the network address and
which part belongs to the node address.
There are 5 different address classes. You can determine which class any IP address is in by examining
the first 4 bits of the IP address.
· Class A addresses begin with 0xxx, or 1 to 126 decimal.
· Class B addresses begin with 10xx, or128 to 191 decimal.
· Class C addresses begin with 110x, or 192 to 223 decimal.
· Class D addresses begin with 1110, or 224 to 239 decimal.
· Class E addresses begin with 1111, or 240 to 254 decimal.
Addresses beginning with 01111111, or 127 decimal, are reserved for loopback and for internal testing on
a local machine. [You can test this: you should always be able to ping 127.0.0.1, whichpoints to yourself]
Class D addresses are reserved for multicasting. Class E addresses are reserved for future use. They
should not be used for host addresses.
Now we can see how the Class determines, by default, which part of the IP address belongs to the
network (N) and which part belongs to the node (n).
· Class A -- NNNNNNNN.nnnnnnnn.nnnnnnn.nnnnnnn
· Class B --NNNNNNNN.NNNNNNNN.nnnnnnnn.nnnnnnnn
· Class C -- NNNNNNNN.NNNNNNNN.NNNNNNNN.nnnnnnnn
In the example, 18.104.22.168 is a Class B address so by default the Network part of the address (also
known as the Network Address) is defined by the first two octets (140.179.x.x) and the node part is
defined by the last 2 octets (x.x.220.200).
In order to specify the network address for a given IP address, the node section is...