Subnetting Nightmare

stevek93

Hi all,

Am doing a project for college at moment. I am trying to determine the number of hosts from the following address block below, if someone could shed some light on this please, simples when you know what you are doing.


206.73.118.0/26

206.73.118.24/29

n0brain3r

Table towards the end of the wiki article http://en.wikipedia.org/wiki/Subnetwork converting to binary and working it out by hand isn't too hard either plus you'll just remember the common ones

stevek93

n0brain3r wrote: »

Table towards the end of the wiki article http://en.wikipedia.org/wiki/Subnetwork converting to binary and working it out by hand isn't too hard either plus you'll just remember the common ones

I am able to convert to binary and AND, just not to sure the route I take in determining the number of hosts is their a video that explain this in more detail?

n0brain3r

Dunno but but the wiki has a whole section on Subnetting and host counts that explains it pretty well. If not look for some CCNA Subnetting videos thats where I picked it up.

stevek93

n0brain3r wrote: »

Dunno but but the wiki has a whole section on Subnetting and host counts that explains it pretty well. If not look for some CCNA Subnetting videos thats where I picked it up.

Will do thanks, theres a great website I wont mention alternative the site with the ship all I wil say :P that is full of video on CCNA.

Redshift

stevek93 wrote: »

Hi all,

Am doing a project for college at moment. I am trying to determine the number of hosts from the following address block below, if someone could shed some light on this please, simples when you know what you are doing.


206.73.118.0/26

206.73.118.24/29

There are 32 bits which make up the address space
For the /26 network you have 26 network bits leaving 6 host bits
So 2 to the power of 6 = 64 addresses in the subnet minus 2 as the first and last address of the subnet, the network address and the broadcast IP cannot be assigned. Leaving you with 62 hosts.

for the /29 subnet you have 3 host bits
so 2 to the power of 3 is 8 addresses minus 2 giving 6 hosts.

wexie

Do a search for magic number subnetting, easiest way I've ever seen it explained.

ZENER

The first subnet uses a 26bit subnet mask, this leaves 6 bits (32 - 26 = 6) for hosts making 64 addresses in each subnet giving 4 unique networks.

These networks will run as follows:

1. 206.73.118.0 to 206.73.118.63 : Network address = 206.73.118.0 : Broadcast = 206.73.118.63 : Hosts = 206.73.118.1 - 62

2. 206.73.118.64 to 206.73.118.127 : Network address = 206.73.118.64 : Broadcast = 206.73.118.127 : Hosts = 206.73.118.65 - 126

3. 206.73.118.128 to 206.73.118.191 : Network address = 206.73.118.128 : Broadcast = 206.73.118.191 : Hosts = 206.73.118.129 - 190

4. 206.73.118.192 to 206.73.118.255 : Network address = 206.73.118.192 : Broadcast = 206.73.118.255 : Hosts = 206.73.118.193 - 254


Your 29bit subnet mask leaves 3 bits (32 - 29 = 3) for host addresses giving a total of 32 networks with 8 addresses in each of which 6 can be used for hosts.

With the address of 206.73.118.24/29 you're in the 4th subnet running from 206.73.118.24 - 31 : 206.73.118.24 is the Network address and 206.73.118.31 being the Broadcast address, hosts run from 25 - 30.


Ken

STEINBERG

Ok so for your first IP 206.73.118.0/26

The /26 is the number of binary 1's is the subnet mask. Lets take the standard Class C subnet mask as an example.

IP address: 206.73.118.0/24
Sub, Mask: 255.255.255.0 if we convert that subnet mask to binary we get 11111111.11111111.11111111.00000000

/24 for the number of 1's known as the network bits, and 8 (0's) known as the host bits. So what does this tell us?
Using the above example, the subnet mask has 4 octets of 8 bits each for a total of 32 bits. To find out how many hosts are on the network
we take the last octet of the subnet mask, that is the 8(0's) our host bits. We use our powers of 2.. so 2^8 = 256. That is 2 to the power of 8.
Gives us a total of 256 addresses on that network, but you need to leave 2 addresses reserved . The first of those is the Network ID
and the second would be the Broadcast address. So 256 - 2 = 254 possible hosts on the above network.

Now moving onto your first example: 206.73.118.0/26

Remember above when i said the /24 was for the number of 1's in the subnet mask? well now you have a /26 so that means there is 26 (1's)
in the subnet mask,,, but how does that work? well lets take a look.

A /26 subnet mask in binary will look like this 11111111.11111111.11111111.11000000
So maybe you ask wtf is happening here? Well its simple realy, in order to sub divide a network or create subnets, we need to borrow
bits from the host portion of the subnet mask. If you remember our first example we had 8 (0's) in the last octect well now we have borrowed two bit's to leave us with 6 (0's) or host bits.

Last octet of /26 subnet mask is .11000000, we have borrowed 2 bits for the network from the host. Now maybe you are asking yourself well how do i find out how many hosts then? Well first you will need to find out how many subnets you have created and thats simple. You take however many bits you have borrowed from the host and you use your powers of 2 again. So we borrowed 2 bits that means.

2^2 = 4 that is we have created 4 subnets by borrowing 2 bits. Now how many hosts per subnet? well thats simple also.
after borrowing 2 bits from the host for the network, we are left with 6 host bits.

2^6 = 64 which means there is a maximum of 64 addresses per subnet with two addresses being reserved in each subnet for the network id and the broadcast address. So that leaves you with 62 usuable host addresses per subnet.

Another quick way to work it out is to remember the base 2 numbering system: 128 64 32 16 8 4 2 1
If we put the last octet of our subnet mask under this it will look like. 1 1 0 0 0 0 0 0

Because we have borrowed 2 bits from the host our subnet mask for the /26 would be 255.255.255.192
Where does the 192 come from? well look above at the base 2 numbering system above the subnet mask
where we borrowed the 2 bits as in the two 1's we add together the numbers above the 1's. 128+64=192.
Whats this got to do with anything well... taking the magic number 256-192 = 64
the number of maximum addresses we can have per subnet. But it also gives us our subnet multipliers.as in

0,64,128,192 each of these numbers is the network id's for our 4 subnets to find the broadcast addresses we simply -1 for the next networkID so,,,

subnet 1 network id= 206.73.118.0 Broadcast id= 206.73.118.63 all host ip addresses are in between these

subnet 2 Network id= 206.73.118.64 broadcast id= 206.73.118.127 all host ip addresses are in between these

Il leave you to work out the next two address ranges yourself and also have a go at doing your second example yourself.
The best way to get used to it is to apply yourself and do it and to possibly if you can teach somebody what you think you know.
If you want come back or even pm me and teach me how you did the second of your examples, trust me you will benefit from it.

Im sorry for all the info, it seems alot to take in but its very simple when you do it a few times. Also remember that computers dont recognise the dots between the octets it is just 32 bits to the computer.

stevek93

Thank you for everyone's help, have ben busy past few days reason I haven't replied.

Basically I am doing it in three steps: determining the number of networks and convert to binary so in the case I need 9 networks so 00001001. 1001 being the 4 bits.

Reverse bits in subnet mask and find your increment, the four bits that we took from the first step were are going to use our 128,64,32,16,8,4,2,1 taking the first four bits 128+64+32+16=240 which give us our new subnet mask 255.255.255.240. Looking at our last bit which is our increment 16 we will use this to determine our network ranges. Say our IP address is 202.10.4.0 we will add 16 each time so 202.10.4.16 202.10.4.32 and so forward. Problem is (I am going to jump ahead here a bit) we cant use the first and last bit correct me if I am working still a bit hazy on this so our IP address now becomes 206.73.118.1 - 206.73.118.14 for our first subnet and 206.73.118.17 - 206.73.118.30 for our second subnet and so on.

STEINBERG

stevek93 wrote: »

Thank you for everyone's help, have ben busy past few days reason I haven't replied.

Basically I am doing it in three steps: determining the number of networks and convert to binary so in the case I need 9 networks so 00001001. 1001 being the 4 bits.

Reverse bits in subnet mask and find your increment, the four bits that we took from the first step were are going to use our 128,64,32,16,8,4,2,1 taking the first four bits 128+64+32+16=240 which give us our new subnet mask 255.255.255.240. Looking at our last bit which is our increment 16 we will use this to determine our network ranges. Say our IP address is 202.10.4.0 we will add 16 each time so 202.10.4.16 202.10.4.32 and so forward. Problem is (I am going to jump ahead here a bit) we cant use the first and last bit correct me if I am working still a bit hazy on this so our IP address now becomes 206.73.118.1 - 206.73.118.14 for our first subnet and 206.73.118.17 - 206.73.118.30 for our second subnet and so on.

When you find out the subnet multiplier you need, yours was 16... then yes you are getting the general idea right.....

The networks would be 0,16,32,48 and so on, remember those are the networks as in the network id's..... so that is already one address taken..

Example: the 16 network.

Network ID: 206.73.118.16

Broadcast address: 206.73.118.31 (one less than the next network ID which is the 32 network)

IP range would be: 206.73.118.17 to 206.73.118.30

You have 16 maximum addresses you can use per subnet but two of those addresses the first one must be saved for the network id and the last saved for the broadcast address. all the usable ip addresses then go inbetween those. Watch the videos in the link below there is about 6, this guy made subnetting so easy to understand for me at least, but understand everybody has there own way of doing it that just works for them. Give them a watch though, its only going to help.

Subnetting, the magic number

stevek93

One more question here I can't get my head around. Am trying determining the number of hosts that can be supported using the address block, 131.107.16.0/20. I think it is more or less the same as before, trouble I'm having is how do I find out how many host do I need? I am saving bits this think instead of borrowing correct?

ZENER

stevek93 wrote: »

One more question here I can't get my head around. Am trying determining the number of hosts that can be supported using the address block, 131.107.16.0/20. I think it is more or less the same as before, trouble I'm having is how do I find out how many host do I need? I am saving bits this think instead of borrowing correct?

A 20 bit SNM means you're now Supernetting. Something that takes a bit of getting used to. This is when you need more than 253 hosts on a single network and also used in BGP and OSPF routing protocols for network aggregation.

One of the quirks is that you can have a valid address with - for example - 192.168.192.255 which might usually be considered a broadcast address until you see the subnet mask that goes with it.

Calculation works similar to subnetting and is easier to do using binary.

Ken

Razzen

ZENER wrote: »

A 20 bit SNM means you're now Supernetting.

Sorry Zener, the 20 bit SNM in this case is not Supernetting..its just subnetting of a class B address. Supernetting for a class B address would involve a mask of less than 16 bits. Sorry for correction..just don't want to confuse OP anymore by introducing anything extra

ZENER

Apologies, I should have seen that from the Class B address. How many host addresses do you need to allow for ?

With the info you've supplied the details would be:

Network: 131.107.16.0
Subnet Mask: 255.255.240.0

Number of Networks: 16
Host per Subnet: 4096

Address Range: 131.107.16.1 - 131.107.31.254
Broadcast: 131.107.31.255

In Binary that would be:

131.107.16.0

Network 10000011 01101011 00010000 00000000
SNM 11111111 11111111 11110000 00000000

= nnnnnnnn nnnnnnnn sssshhhh hhhhhhhh

n= network ID
s= subnetwork
h= hosts

Hope this helps.

Ken