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Home > Archive > CCNA > October 2001 > subnetting
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| Hello
On page 141 of Todd bood (2nd edition) on # 3 he says "what are the valid subnets? 256 - 224=32", my question is where he got the 256 from?
TIA | |
| bills_101 2001-10-06, 3:05 pm |
| 2 to the power of 8 gives you 256. | |
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| Thx, so it is ALWAYS 256 - the mask # ? | |
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| Go here for all you need to know about IP addressing and subnetting. It is the exact document I use when I teach it... and my students all get subnetting in under 20mn 
Hope this helps...
http://www.mcsefreak.com/subnetting.htm | |
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| depamo 2001-10-09, 1:16 am |
| There is no Subnet Mask of 256, 256 is
100000000
which is 9 digits, last I herd an 'octet' was 8 digits. The highest subnet mask you can get in any octet is
11111111
or 255.
Rest are not legal, as a matter of fact the only legal Subnet Masks are
255, 254, 252, 248, 240, 224, 192, 128, 0
Thats it, memorize these, essential!! | |
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| Depamo is right in saying that there is no legal subnet of 256 however the method you are being taught is perfectly legal - it's just a different method.
256 is simply the range of bits allowed i.e. 0 to 255 - giving a range of 256 possible combinations.
I think the question which is being asked is what is the possible range of subnets when using a mask of 224 (they possibly gave you a bit more info such as 255.255.255.224 or /27).
To find this out you do use the formula 256 - subnet mask, in this case 256 - 224=32. This is the sybex way not Cisco - but easy to use and very reliable. It doesn't really require any knowledge of binary (although it does help).
This isn't the answer though, it simply means that each subnet has a theoretical range of 32 addresses which can be addressed, however 2 of these can't be used (broadcast and 0 the subnet address).
The possible subnets in this case then are all of the multiples of 32, i.e x.x.x.32, x.x.x.64, x.x.x.96 etc. In the first answer then you have a host range of x.x.x.33 to 62, with 63 being the broadcast address for that subnet and 32 the subnet address. The next subnet address is x.x.x.64 etc.
Hope that helps. |
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