From: ccie2be (ccie2be@nyc.rr.com)
Date: Fri Jun 24 2005 - 17:15:13 GMT-3
Dirk,
It might help if you remember the function of a subnet mask - it tells a
networking device which portion of the ip address belongs to the network and
which portion belongs to the host.
Without a netmask and assuming classless networking, how would a networking
device know which portion of the ip address represents the network address
and which represents the host address.
For example, consider 122.189.12.196
In this ip address, which bits represent the network address?
122 ?
122.189 ?
122.189.12 ?
122.189.12.192 ?
Without a subnet mask there's no way to know.
It could be the first 8 bits, the first 16, the first 24 bits, and even the
first 26 bits. Actually, it could be any number of bits between 8 and 31.
(192 = 1100 0000 in binary)
In your example, the first 2 octets (16 bits) of the net mask are 255 255.
In binary, that's equal to 1111 1111 and 1111 1111.
Remember that a one in the net mask means the corresponding bit of the ip
address is part of the network address. So, back to your example - since
the first 16 bits of the netmask are all one's, that means that the first 16
bits of your ip address are part of the network address.
Now, what's 50 in binary?
50 is equal to 32 + 16 + 2 which in binary is 0011 0010.
So, the problem with this is that not all the one bits in the netmask are
consecutive but they have to be.
IOW, when u look at a netmask in binary the first 8 bits will always be 1
and the last bit will always be zero and altogether there are 32 bits.
So, the questions becomes where between the 8th bit and the 32nd bit in the
netmask do the bits change from being one to being zero. Where the bits
change from being one to being zero is the dividing line between the network
portion and the host portion of the ip address - all the one bits on the
left represent the network portion and all the zero bits on the right
represent the host portion.
It actually isn't all the hard once it "clicks" but it is fundamental to
understanding ip networking. so, make sure you're 100% clear on this.
HTH, Tim
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Stewart, Dirk
Sent: Friday, June 24, 2005 3:22 PM
To: ccielab@groupstudy.com
Subject: Subnet Question? Help
Is this a valid subnet and where and how would I use it. Does ISP use
this kind of subnet.. If this is possible how much networks and host
could I get from it...
172.16.26.49
255.255.50.0
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