Thanks Kurt,
Now i understand, the 4 octet of the network address ( i was confusing the
term ip address and network address) with a /25 mask can only be
00000000 or
10000000
ie 0 or 128
"Kurt" wrote:
>
> "Larry" <> wrote in message
> news:C95BE911-BA7F-44CF-B4BF-...
> > According to the microsoft testing software, the following ip addresses
> > using
> > VLSM are
> > invalid.
> > Im still trying to understand why they would be invalid. Can anyone offer
> > a
> > quick explanation, or point me to a web site explaining this.
> >
> > invalid class c ip addresses
> > 199.205.15.8/27
> > 199.205.15.64/25
> > 199.205.15.130/26
>
> 199.205.15.8/27 - This is a valid IP address, it is NOT a valid network
> address. Think of it this way: every time you add a bit (1) to the subnet
> mask, you cut it in half. So a /24 (mask 255.255.255.0) has 256 addresses
> the networks are 192.168.0.0/24, 192.168.1.0/24, 192.168.2.0/24, etc.
>
> If you add a bit, making the mask /25, your networks are on boundaries of
> 128 so your network addresses are 192.168.0.0/25, 192.168.0.128/25,
> 192.168.1.0/25, 192.168.1.128/25, etc.
>
> a /26 would be at boundaries of 64 and a /27 at 32.
>
> So valid network addresses for a /26 are 192.168.0.0/26, 192.168.0.64/26,
> 192.168.0.128/26, etc.
>
> Valid networks with a /27 mask are 192.168.0.0/27, 192.168.0.32/17,
> 192.168.0.64/27, etc.
>
> A lot of times, it's not the subnetting that gets you, it's understanding
> binary arithmetic. Just as in base 10, when we get to the last digit (9) in
> a column and add one more to it, we zero the column and carry to the next
> column to the left. So the opposite is shifting to the right. In base 10,
> when you shift everything one column to the right, it's the same as dividing
> by 10. In binary, shifting one column to the right is the same as dividing
> by 2 (0100 binary = 4 decimal, if you shift right one - 0010 binary = 2
> decimal or 4 divided by 2). Since the subnet mask defines the number of host
> bits, every time you add another "1" to the right, you cut the number of
> hosts in half, and each time you reach the number of hosts and add one more,
> the hosts bits are all zero'd and the network is incremented.
>
> IP address 10.10.10.255/24 - in binary 00001010 00001010 00001010 11111111
> S/N mask 255.555.255.0 11111111 11111111 11111111
> 00000000
>
> so if you add one to the IP address you get
> 00001010 00001010 00001010 11111111
> + 00000000 00000000 00000000 00000001
> -------------------------------------------------
> = 00001010 00001010 00001011 00000000
>
> in decimal this is 10.10.11.0, the next network.
>
> ....kurt
>
>
>
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