Netmasks: Was New AS/400 Update - TCP/IP Problems

Booth,

You are right, 255.255.255.255 is not usable.  Was a
typo on my part.  I meant to say 255.255.255.0.  The
reason 255.255.255.255 is not  usuable is becaue it
means only your IP is in the sub net.  But each sub-net
has 2 addresses that can not be used.  The lowest which
is .., er, something I forget, the router maybe, and the
highest which is the broadcast.

Which would leave -1 usuable addresses.

And you got caught by the same thing I did.  The
netmask does not tell your what your allowable
ip addresses are.  Your IP address and the net-mask
states what the allowable IP range is for the netmask.

Consider this.  I have an IP address of, say, 10.0.0.3.
My netmask is 255.255.255.240.  My sub-net goes
from 10.0.0.1 to 10.0.0.15 with 1 being the one I
forget and 15 being broadcast.  The reason is: the bits
of 3 are: 0 0 0 0 0 0 1 1
The bits of the 240 in my netmask is: 1 1 1 1 0 0 0 0
The one's in the netmask says that for the same bit
position in my IP, they can not change.  Since my 4
high bits are 0's, any IP in my sub net must also have
0's in the 4 high bits in the last byte..  Which gives us:
0 0 0 0 0 0 0 1 through 0 0 0 0 1 1 1 1

On the other hand, if my IP address is 10.0.0.19 and
my netmask is 255.255.255.240 my sub-net goes
from 10.0.0.17 to 10.0.0.31 with 17 being the one
I just can't remember and 31 being broadcast.
Bitmask of 19 is 0 0 0 1 0 0 1 1.  Again, 240 is
1 1 1 1 0 0 0 0.  The 4 high bits in my IP can not
change, which are 0 0 0 1.  So the last byte of
my subnet goes from 0 0 0 1 0 0 0 1 though
0 0 0 1 1 1 1 1.

This goes for all the bytes in the subnet.

Regards,

Jim Langston

boothm@earth.goddard.edu wrote:

> "255.255.255.255"? is that a usable mask?  That  seems to me to mean there
> is only 1 valid address for the whole network. and that address is
> xxx.xxx.xxx.0?

<SNIP>

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