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# Number of Available subnets?

Member Posts: 76 ■■□□□□□□□□
Does anyone know the number of available subnets for these network IDs?

Network ID
207.209.68.0 /27
131.107.0.0 /20
10.0.0.0 /13
208.147.66.0 /25

These are my answerse, please let me know if it's correct. The reason why I'm asking is because on the answer sheet it shows something else. Any help would be appreciated. Thank you.

207.209.68.0 /27 - my answer is 8
131.107.0.0 /20 - my answer is 16
10.0.0.0 - my answer is 24
208.147.66.0 /25 - my answer is 8

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Member Posts: 2,111 ■■■■□□□□□□
207.209.68.0/27 has 8
131.107.0.0/20 has 16
10.0.0.0/13 has 32
208.147.66.0/25 has 2
Networking, sometimes i love it, mostly i hate it.Its all about the \$\$\$\$
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Member Posts: 3,352 ■■■■□□□□□□
207.209.68.0 /27
131.107.0.0 /20
10.0.0.0 /13
208.147.66.0 /25

207.209.68.0 /27 = Class C = Default of /24 = 3 available bits for subnets = 2^3=8 subnets

131.107.0.0 /20 = Class B = Default of /16 = 4 available bits for subnets = 2^4=16 subnets

10.0.0.0 /13 = Class A = Default of /8 = 5 available bits for subnets = 2^5=32 subnets

208.147.66.0 /25 = Class C = Default of /24 = 1 available bit for subnets = 2^1=2 subnets
“For success, attitude is equally as important as ability.” - Harry F. Banks
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Member Posts: 76 ■■□□□□□□□□
Thank you guys! I understand now. Again THANKS!!!
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Member Posts: 326
Wow, this really helps alot. Thanks All!

Although I continue to struggle with subnetting, I am still trying my best and this really does help.
Thanks again.
A+, MCP(270,290), CCNA 2008.
Working back on my CCNA and then possibly CCNP.
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Member Posts: 3,352 ■■■■□□□□□□

http://www.techexams.net/forums/viewtopic.php?t=16824
“For success, attitude is equally as important as ability.” - Harry F. Banks
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Member Posts: 68 ■■□□□□□□□□
I dont see how or what the /20 is after an IP address WTF does it mean.

I'm lost on this issue it's like reading a different language or something.

Update:
After using google and finding http://www.cisco.com/warp/public/701/3.html#ustand_subnet
I now understand that the number just represents the number of bits used to make the subnet mask.

Example: /20
In English means: 11111111.11111111.11110000.00000000 =255.255.240.0 for the Subnet.

Example: /27
In English means: 11111111.11111111.11111111.11100000 =255.255.255.224 for the Subnet.

Oh they have a cool link has a nice PDF option off to the right.
Don't tell co-workers your going for a Cert that they don't have. They may think your trying to take their job.
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Member Posts: 3,352 ■■■■□□□□□□
It basically means the subnet mask will have 20 consecutive 1s.
1111111.11111111.1111.0000.00000000
255.255.240.0

The bits are as follows 128 64 32 16 8 4 2 1

Since the 3rd octet is 11110000, that means the 4 left most bits are turned on. Therefore we take 128 + 64 + 32 + 16 = 240.

__________________________________________________________

So a real world example? Well I just created a bunch of static routes the other day to have ISA talk to a bunch of internal VLANs, so I'll talk about that.

The client had a bunch of internal VLANs on different networks such as 172.16.0.0/16, 172.17.0.0/16, 172.18.0.0/16, and so on.

/16 means that the subnet mask is 16 consecutive 1s
1111111.1111111.00000000.00000000
255.255.0.0

The IP is 172.16.0.0

Because the network bits on a subnet mask are all consecutive 1s on both the 1st and 2nd octet, if you change anything in the 1st or 2nd octet of an ip, it'll belong on a different network. This can be proven by using the ANDing process.

172.16.0.0
vs
172.17.0.0

1st octet will obviously match

16 - 00010001
vs
17 - 00010000

Since everything matches but the last bit, we can see that they are on a different network. This is where the /16 plays in. It gives us the subnet mask which allows us to see how many bits to use to determine if ips are on a different network by using the ANDing process.

Does this make any more sense or am I mixing 10 different languages together and trying to form a new language that you've never heard of?
“For success, attitude is equally as important as ability.” - Harry F. Banks
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Member Posts: 230
royal wrote:
207.209.68.0 /27
131.107.0.0 /20
10.0.0.0 /13
208.147.66.0 /25

207.209.68.0 /27 = Class C = Default of /24 = 3 available bits for subnets = 2^3=8 subnets

131.107.0.0 /20 = Class B = Default of /16 = 4 available bits for subnets = 2^4=16 subnets

10.0.0.0 /13 = Class A = Default of /8 = 5 available bits for subnets = 2^5=32 subnets

208.147.66.0 /25 = Class C = Default of /24 = 1 available bit for subnets = 2^1=2 subnets

Good explanation, royal!

Not sure if this will help clear up anything, but some info in a thread I started a few weeks ago (http://www.techexams.net/forums/viewtopic.php?t=24568) explains some of the information discusses above in a different way -- not sure if it'll help or not, but worth looking over if it'll make things 'click'