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TOPIC: Subnets and broadcast address

Subnets and broadcast address 14 years 9 months ago #3529

  • thorpe
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I am trying to understand subnets a little further so my question may be a little stupid.

If I have an network IP address say and I subnet this with (

If I am given an Ip address I can figure out its network address and subnet address easily for example is in the network of

so how do I know what the broadcast address is for that network and how can I figure out when the network addresses change for example is in the network of

hope some one can help

Re: Subnets and broadcast address 14 years 9 months ago #3532

hmmm broadcast address is the last address in that subnet...
but i think somethin is wrong with ur subnettin..coz the netwk is a class c...from your no of network bits...there is only 22...u have not borrowed any netwk bits....
so i dont think u have subnetted coz the number of subnets is equal to 2^n - 2
n being the number of bits u borrowed from host side,,,...hmmm tts what i ermember

Re: Subnets and broadcast address 14 years 9 months ago #3536

Nope, the /22 is classless.. what you're actually doing is supernetting..
by borrowing from the network portion of the address..

either that or....

you only have one subnet in this case
the host range is - 3.254
broadcast is

I have confused myself, Chris will step in on this one.
Sahir Hidayatullah. Staff - Associate Editor & Security Advisor

a slight error 14 years 9 months ago #3549

  • thorpe
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I was actually meant to put a class B address Just happit that I used a class c.

lets say the IP was (I think that would be the class B range).

If the IP address was the above Is my example as gives 10 for the host and 22 for the network

What I am try to figure out is given an network IP address and a subnet mask how can I work out the subnet ranges.

Sorry if this a little confusing, I am still learning

Re: Subnets and broadcast address 14 years 9 months ago #3552

Lets take a simple example.. with a subnet mask of

Now how do we find our ranges ?
We do 256 - 224 (the octet we are subnetting) this gives us
32 as our block size.. in other words, all subnets will be in multiples of 32.. so the subnets will be -- this is subnet 0 and is not valid in Cisco exams

Now the broadcast addresses for each subnet will be one less than the next subnet address.. so for the broadcast will be (since 10.64 is the next subnet address)...
your valid hosts are all the ones between the subnet address and the broadcast address.. so for the subnet, valid hosts will be between 1 - 62 (63 is broadcast).

Remember, broadcast address is one less than the next subnet address.
Sahir Hidayatullah. Staff - Associate Editor & Security Advisor

Re: Subnets and broadcast address 14 years 9 months ago #3553

  • Chris
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Hi Thorpe,

I've got the answer for your question right here:) You happen to be in luck because I only just recently answered a member's question similar to yours for, so here's the actual answer ... read through it as I'm sure you will find it insightful!

Variable Length Subnet Masks (VLSM) can require some serious number crunching to figure out when they start getting complicated and there are many ways to help make the process more simple.

In my experience of teaching the CCNA course, I have found that every person essentially will find their own way of figuring the fastest method to calculate them as long as they work on it hard enough.

What I can do, hoping to help you, is show you the method I use, so you can then take it and see how well it suites you!

Before we dive into the deep waters, let me show you how I calculate network ranges from given subnet masks:

Assume network and subnet mask

I first calculate each subnet's range: 256-240=16 IP's per subnet

This means we have the following subnets:

As we said, for each subnet, we have 16 IP's this includes network and broadcast addresses. Looking at the above table, I already have the network address for each subnet, so let's reveal the broadcast address:

0 - 15
16 - 31
32 - 47
48 - 63

The logic behind this is quite simple: take the next network and subtracted one from it to get the broadcast address

Now that we have the network and broadcast address, its easy to calculate the range of VALID IP addresses:

0 (1 to 14) 15
16 (17 to 30) 31
32 (33 to 46) 47
48 (49 to 62) 63

You can clearly see that everything between the network and broadcast address of each subnet, is a VALID IP address.

This method is perhaps the best and fastest way to calculate networks and IP ranges from different subnet masks.

One step up, is VLSM, where we are given any possible subnet mask and we need to calculate the subnets and IP ranges.

For this, I have created a nice table which is available on my site, under the 'supernetting/CIDR' topic ( ). The chart shows all possible subnet mask combinations and the corresponding number of networks and hosts calculated for each one.

A closer look at the table will reveal a nice pattern that is repeated:

Subnet Bits Subnet mask

Notice the subnet mask for /17 and /25, /18 and /26.
Both /17 and /25 have the 128 decimal number in their subnet mask, while /18 and /26 have decimal 192. The only part that changes in the pattern is the location of the last 'non-zero' decimal number.

Considering that /8, /16 and /24 have only 255 and 0 decimals, so whenever I see a number between these ranges, I use the closest of the three as a reference point.

For example, /19 is a standard 16 bit subnet mask, with an additional 3 bits. If you memorise the following, then it becomes an easy task translating VLSM numbers to real decimal subnet masks:

1 = 128
2 = 192
3 = 224
4 = 240
5 = 248
6 = 252
7 = 254

So, /21 is (2x8)+ 5, that is !

To figure out the subnet masks here, simply do the following calculations:
256-248=8 That's 8 IP's per subnet. We need to reserve 2 IP's (network and broadcast addresses), leaving us with 6 valid IP's per subnet.

From all the above calculations, the hardest part is memorizing the last table, which is only 7 entries. From there on, it's a matter of using the quick method I just showed, to calculate your subnets and their ranges!

Hope that solved your problem!
Chris Partsenidis.
Founder & Editor-in-Chief
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