by Matthew G. Naugle
Wiley Computer Publishing, John Wiley & Sons, Inc.
ISBN: 0471196568 Pub Date: 11/01/98
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Illustrated TCP/IP
by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568 Pub Date: 11/01/98 |
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Subnetting confiscates unused bits, allowing for more efficient use of an address. Subnetting allows for more efficient use of addressing space and lowers the number of routes in the Internet routing tables. The bits are taken away from host assignment and given back to identify a subnet of a network address. The slide depicts this. The subnet is a real network. It is a subnet under the network number. With subnetting a Class B address, we can take any amount of the bits in the third byte, or 6 bits of the fourth byte (1 through 8 bits; they should be contiguous, starting from the left) of the IP address and make them part of the network number (a subnet under the network number). The format of the IP address would now be: <network number, subnet number, host number>. For example, if the address assigned to a particular host is 130.1.5.1, the network portion would be 128.1 and the host portion would be 5.1. With subnetting (assuming all 8 bits of the third field were consumed for a subnet address), the address would be defined as network number 130.1 and subnet 5, with a host ID of 1.
Subnetting a Class B address is easy when you subnet the entire third octet. However, it becomes difficult when you subnet only a portion of the third octet. Suppose the first 5 bits (starting from the left; they should start from the left and remain contiguous going to the right) are reserved in the third field for assigning subnet numbers. What subnets do we have now? Convert those first 5 bits of that octet to binary. All five of those bits are now assigned to the subnet number and may not be used for host IDs. Five bits yields 32 subnet numbers (2n5). Now, the big challenge: Identify those numbers!
If we start from the left and go 5 bits to the right, we get X.X.11111000.X as a network number (we don’t care what is in the X). The binary numbers are taken literally and will yield subnets in multiples of 8 (8 is the first binary bit set to a 1). This gives us 0, 8, 16, 24, 32, 40, 48, 56, 64, 72, 80, 88, 96, 104, 112, 120, 128, 136, 144, 152, 160, 168, 176, 184, 192, 200, 208, 216, 224, 232, 240, 248.
You really must completely understand binary before heading into this area.
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