Help Needed in Subnetting 2

[garyrose]

I'm trying to teach myself for my CCNA and I ran across a problem in subnetting, I'm not sure I fully understand it.
Here's the question:
If you network number is: 70.12.12.0 and your mask is 255.255.255.192
1. How many subnets do you have?
2. What is the first subnet?
3. What is your last subnet?
4. How many host do you have on each subnet.

What I understand is that the address is a class A address, so would it be right to think that I have a 18 bit subnet mask? What confusing me is the question that "the network number is" 70.12.12.0. Would it be that the netwotk
number should be 70.0.0.0 and the host is 0.12.12.0

 

[Guest_imported]

with a subnet mask of 255.255.255.192 (/26), you have 4 subnets with 62 hosts each. Here's the breakdown:

subnet 1: 1-62
subnet 2: 65-126
subnet 3: 129-190
subnet 4: 193-254

If your subnet mask was 255.0.0.0, then what you said about the network address being 70.0.0.0 would be true. But with a subnet mask of 255.255.255.192 then your network addresses are:

subnet 1: 70.12.12.0
subnet 2: 70.12.12.64
subnet 3: 70.12.12.128
subnet 4: 70.12.12.192

 

[mntlfngrs]

subnetted as class A:
262144 networks
62 hosts
first 70.0.0.0
last 70..255.255.192
our on 70.12.12.0

I think they want it subnetted as a class C. IE.. provider bought 70.0.0.0 and you got 70.12.12.0 from them.

4 networks
62 hosts
first 70.12.12.0
last 70.12.12.192

This is a helpfull chart if it will come out right when posted.
first line slash notation.
second line is the mask in dotted decimal
third line is 2 to the nth notation
fourth line is dec. equivalent to binary value (exept 1st column sepperated by ||, is part of third oct.)
fifth line is to help compute host/networks.

/24 || /25 | /26 | /27 | /28 | /29 | /30 | /31 | /32
0 || 128 | 192 | 224 | 240 | 248 | 252 | 254 |255
2^8 || 2^7 | 2^6 | 2^5 | 2^4 | 2^3 | 2^2 | 2^1 | 2^0
255 || 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1
1 || 2 | 4 | 8 | 16 | 32 | 64 | 128 |255

So a /26 mask (x.x.x.192) would have 64-2 hosts and 4 networks. "Be all and you'll be to end all.
Life can be a real ball.
State of mind!"

 

[garyrose]

Just wondering, How would you be able to tell if you have a 18 bit subnet verse a 2 bit subnet. say I wanted 250,000 subnets with 62 host each,from a class A address as 70.0.0.0 with a subnet mask 255.255.255.192. Wouldn't it creat that many subnets?

 

[GlenJohnson]

Subnetting has got to me the most esotaric subject I have ever come accross.

http://www.learntcpip.com

has some good stuff on it, but I just can't seem to get the grasp. I need to learn subnetting so I can put my engineers on a different subnet to help network performance. Anybody got any other places that helps learn subnetting? Thanks. Glen A. Johnson
Microsoft Certified Professional
gjohn76351@msn.com

"Beware of false knowledge; it is more dangerous than ignorance."
George Bernard Shaw (1856 - 1950) English dramatist, critic

 

[adam108]

To truly understand the concept of a subnet and masking, you have to visualize an ip address not as 4 numbers seperated by dots but instead as 32 bits. It isn't necessary to grasp the values at first but basically each bit represents whether a multiple of 2. The values are:

128 64 32 16 8 4 2 1

so

1 0 1 0 1 0 1 0 is represented for us as 170

Subnet masks are used to indicate how many of the 32 bits of an ip address are used to represent the Network Address. The remaining bits are used to represent the Host ID. If you think of an IP Address as a mailing address you can start to see the significance. For example, Given the address 123 45th St

If the street address were written as one word,
12345th St.

We could easily get confused as to which part was the House number (HOST ID - 123) and which part was the Street name (Network ID - 45th Street)

Of course in street addresses, we just use a space to avoid situations like above.

In the world of IP we don't have the luxury of spaces, so we use the subnet mask to indicate how many bits from left to right we should count before we find the HOST ID (house number).

To understand how the subnet mask is used to indicate how many bits you just need to know that each number in the quartet represents a number of bits, used from left to right

255 254 252 248 240 224 192 128 0
8 7 6 5 4 3 2 1 0

So 255.255.255.128 uses 8 bits + 8 bits + 8 bits + 1 Bit to represent the network ID = 25 bits or /25.

I use the above examples to train people at my workplace and I hope this helps. mntlfngrs gives a great graph of how to apply this above in this thread and it is worth recording for reference. If this isn't helpful let me know and I won't burden others with it again.

 

[yarivco]

Hi there
You can download this subnet calc( IP Subnet Calculator)
that can be very helpful to understand .
Download it from

http://www.boson.com/promo/utilities.htm#utilities

 

[CCIEWANNABE]

70.12.12.0 converted to binary equals ………………01000110. 00001100. 00001100.00000000
Mask is 255.255.255.192 converted to binary bits 11111111.11111111.11111111.11000000[/red]

0 = TYPE OF NETWORK (A,B,C,D,E) [/B]0[/B]nnnnnnn
N = NETWORK Class
S = Possible subnets
H = Host values

Subnet bit mask: 0nnnnnnn.ssssssss.ssssssss.sshhhhhh[/red]
Subnet Bits : 18 = s
Host Bits[/red] : 6 = h

There are 32 bits to every IP address. The mask bits tell us the subnet value.

255.255.255.192 = 0nnnnnnn.ssssssss.ssssssss.ss= 26 bits and 18 are for subnet use. Every IP address has four fields called an octet. Octet is sometimes used instead of the term byte to avoid confusion. Every Octet has eight bits. 70 equal a decimal value and is a full octet of eight bits. Remeber every octet has eight bits. The binary eight-bit value is 01000110and the decimal value equals seventy.

Two rules to remember:

(2^N) - 2 = the number of subnets or hosts[/red] in a subnet
N = the number of bits used in the mask
Example:
We have hhhhhh[/red]6 possible bit values
(2 to the power of six) - two = 64 possible host values

We have 26 possible bit values 0nnnnnnn.ssssssss.ssssssss.ss
(2 to the power of 18) - two = 262144
All values of ones is consider a broadcast. 0nnnnnnn.ssssssss.ssssssss.sshhhhhh[/red]
If all H=hosts[/red] values were all ones that would be your braodcast value.

Each octet has 8 bits and each bit has a decimal value. Counting from right to left.

128 = 10000000 each value is halved henceforth
64 = 01000000
32 = 00100000
16 = 00010000
08 = 00001000
04 = 00000100
02 = 00000010
01 = 00000001
For example:
70 = 01[/red]00011[/red]0.....64+4+2= 70





Route once; switch many

 

[CCIEWANNABE]

One additional note. The mask value given is called CIDR (Classless InterDomain Routing) 255.255.255.192 or /26

Classful addresses are only one of three values
Class A = /8 or 255.0.0.0 or 0nnnnnnn.hhhhhhhh.hhhhhhhh.hhhhhhhh
Class B = /16 or 255.255.0.0 or 10nnnnnn.nnnnnnnn.hhhhhhhh.hhhhhhhh
Class C = /24 or 255.255.255.0 or 110nnnnn.nnnnnnnn.nnnnnnnn.hhhhhhhh
There is a special notation (Shorthand) for your mask 255.255.255.192 which equals a classless address and the notation equals /26
http://sparky.freesoft.org/CIE/Course/Subnet/202.gif]

Link:
[URL unfurl="true"]http://sparky.freesoft.org/CIE/Course/Subnet/202.htm[/URL] Route once; switch many