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what is /24 in the subnet?

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by thinknologist 1 view

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Need help. Router port has following details

EO: 172.16.2.1 Mask:255.255.255.0/24

What does /24 stands for?

Best regards

4 replies

by tinscore

I'll make the assumption that you have at least basic knowledge of TCP/IP addressing. First I offer a summary, the /? at the end of a subnet ID is an agreed upon (lazy but) effective method of identifying amount of subnets and their hosts but simplified, it is the amount of ones. ???? NOW what the heck DOES THAT mean???? ok....for each octet there are 8 characters (ones or zeros) per octet (hence octet)....so if the subnet mask is 255.255.255.192 that would be a /26 because 255 in octal is 11111111 and 192 is 11000000 so in octal the subnet mask would look like 111111111.11111111.11111111.11000000 with the result being four subnets with 64 addresses available in each for a class "C" network. To illustrate, consider a packet with the destination address 204.13.31.69 (as an example) when the above mask is applied, the first two bits in the forth byte are 01. The router concludes that the destination is in the second subnet (which has the address of 204.13.31.64). The target host is 69. Had the mask not been applied, the router could not pick the interface corresponding to the correct subnet from the four subnets. Hope I helped.

by mrbill-

To expound (SP?) on Tinscore's answer

172.16.2.1 Mask:255.255.255.0 is the same as 172.16.2.1/24 the /24 is for Classless Inter-Domain Routing (CIDR) an advanced subnetting scheme.

It allows more efficiant use of the Class B and C IP addresses and helps shorten routing tables.

"A great deal of space can be saved in a routing table by using CIDR notation instead of the traditional subnet mask dotted decimal notation. For example, if you replace 255.255.240.0 with /20, space is saved and the exact same information is conveyed. When you multiply this saved space by the number of routes in a routing table, which could be thousands, a tremendous amount of space is saved." From "TCP/IP Jumpstart" Andrew G. Blank Chap 10.

I still do not have a strong handle on subnetting yet myself, but I think I got this right, agreed?

by lordinfidel

You both are right.. I will just break it down even dumber.

The / after an IP is the amount of "bits" or spaces that is equal to the dotted decimal subnet mask. Also known as CIDR Notation.

Let's say your IP is 192.168.1.1
And you have a subnet mask of 255.0.0.0
Which takes up the first 8 bits of the mask.
You can effectively say that you have a /8
It will then be written like this
192.168.1.1/8

Now there are 8 bits per octect

8bits.8bits.8bits.8bits

If you add them up from left to right you get
8 16 24 32
8bits.8bits.8bits.8bits

So when you see 192.168.1.1/24
it represents 255.255.255.0

Now if you see 192.168.1.1/25 that means that
the 1st 3 octects have been filled plus the 1st bit of the last octect. So your netmask would now look like this:
255.255.255.128 and it goes down the line.

just remember the placement of each bit.

I would reccomend writing it down. I will post a chart below for you.

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