[Gllug] subnetting

[Ian Northeast]

Michael Jenson wrote:
> Hello
> 
> Well OK I should know this I feel, but I don't! 
> Can someone send URL or explain why the first range of IP's 
> in subnetting is not used and what would make it possible to
> use it. Example:
> Taking 7 bits away makes for 8 subnets. How can I make the 
> first subnet usable for a customer. 

Well as others have said your question is not clear.

Taking 7 bits away from *what* makes for 8 subnets? Taking *three* bits 
away from the host portion of an address and adding it to the network 
portion will split one subnet into 8.

Within a subnet, only the first (network number) and last (broadcast) 
addresses cannot be used for hosts. All 8 subnets can be used.

E.g you have a network 192.168.50.0/24. This has a subnet mask 
255.255.255.0, 24 bits in its network address and 8 bits in the host 
address. 192.168.50.0 and 192.168.50.255 cannot be used for hosts, the 
remaining 254 addresses can.

You can split this into 8 /27 networks by using a subnet mask of 
255.255.255.224, so you have 27 bits in the network portion of the 
address and 5 in the host part. Your 8 networks are 192.168.50.0 - 
192.168.50.31, 192.168.50.32 - 192.168.50.63, etc. Again, the first and 
last addresses in each subnet cannot be used, i.e. 192.168.50.0, 
192.168.50.31, 192.168.50.32, .. So you lose 14 addresses compared to 
using the one /24. But the first subnet can be used - 192.168.50.1 - 
192.168.50.30 are all available for hosts.

There's a perl script called "ipcalc" which is often found on Linux 
systems by default and can easily be found by googling which makes 
subnet calculations very easy. It shows you all aspects of the subnet in 
both decimal and binary and so it's quite a useful study aid too.

Regards, Ian

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