Subnet Masking Made Easy

Don't Panic Subnet Masking Don't Panic

The first thing about networking in general is: Don't Panic!

Subnet Masking is an art rarely performed due to the difficulty in understanding exactly what you are *doing*. Ideally, it is the ability to break down (usually a Class C IP block) a range of numbers in to smaller sub-networks (Hence the subnet part of subnet masking). Well, obviously, okay maybe not, the rest of the world does not see each subnet as its own seperate, logical entity unless it is run through a filter or a mask. This effectively allows other computers know that this is a LAN separate from the rest of the range of numbers.

Realize, once you start subnetting, you start losing IP addresses very fast. Recognize that a.b.c.0 and a.b.c.255 are reserved. The first for the physical line/host and the latter for broadcasting. When you start subnetting, you imediately lose the first and last subnet for similiar reasons. Also, the first and last numbers in the new sub-network are reserved as previously stated for line and broadcasting. This leads to an atrocious waste of usable IP addresses. However, if you have a Class C block to spare (give me a holler) this is one way to break it down.

Subnet Mask Binary Usable Subnets Binary of Subnets Usable IP/Subnet
128 10000000 0 00000000 0
192 11000000 2 00000010 62
224 11100000 6 00000110 30
240 11110000 14 00001110 14
248 11111000 30 00011110 6
252 11111100 62 00111110 2
254 11111110 126 01111110 0
255 11111111 No Subnets 00000000 254

If you want, say 5 sub-networks, you would break it along the x.x.x.224 subnet mask, for a total of six sub-networks and the last subnet would be idle and totally unused.

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Copyrighted© 1996-2000 by Timothy Mui

This page was last modified for human consumption on Aug. 23, 2000