IP Address syntax questions

Posted on 2013-12-02
Last Modified: 2013-12-03
I need help understanding the following:

I've installed PC's and worked with IP but I've never seen the syntax of a slash.

This is from a router diagram.
Question by:brothertruffle880

Expert Comment

ID: 39690815
The syntax with a slash is the CIDR syntax that represent the range of IP address
Here is some URL that convert range of IP to CIDR or reverse
LVL 26

Expert Comment

ID: 39690828
the /24 says that you have 256 addresses available or that you are using the netmask of

Some samples:

/20     4096

/21     2048

/22     1024

/23     512

/24     256

/25 128

/26 64

/27 32

/28 16

/29 8

/30 4

Author Comment

ID: 39690902
Jeepers.  I've worked on IP addresses for a while and neverheard of CIDR.
How does one arrive at 256 addresses if you have a "/24"?  I'm trying to understand the math.
LVL 28

Accepted Solution

Jan Springer earned 250 total points
ID: 39690910
and to be more specific:  the number after the / indicates the number of bits in the mask:

/24  11111111.11111111.11111111.00000000

so, the first three octets much match exactly and the last octet can be anything from 0 to 255.

and the bits in the mask identify the network start (also called a boundary) and the network end (also called the broadcast).
LVL 27

Assisted Solution

tliotta earned 250 total points
ID: 39692261
Earlier IP addressing used a "class" system to distinguish and separate 'network' addresses from 'host' addresses. The boundaries used for the separation were generally at octet boundaries. E.g., class A generally used the first octet as the 'network' address, leaving the last three octets available for 'host' addresses; that means that a class A network could have up to 16,777, 214 hosts. Class B generally used the first two octets, and class C used the first three octets. Those last two classes have successively fewer possible hosts per network because the network portion takes up more bits, each reduced by a factor of approximately 256.

Using the CIDR (Classless Inter-Domain Routing, established in 1993) notation, a class A network would be /8, class B would be /16, and class C would be /24. That's only part of it, though, because those classes also have requirements such as for bit patterns in the left-most octet. For class A, the left-most bit in the left-most octet must be '0'. For class B, the left-most two bits must be '10'. And for class C, the first three bits must be '110'.

In any case, the CIDR notation effectively eliminates the "classes". Instead, the 'network' address is specified by giving the number of bits in that portion, and the number does not have to fall on an octet boundary.

There is quite a lot that can be learned about what CIDR makes available. A Google (or other search engine) query for [ what is cidr ] will bring up a number of good pages for study.


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