[ih] "Subnet"

[Bernie Cosell]

On 10 May 2012 at 17:17, Larry Sheldon wrote:

> People with forever talk as if the subnet number was the third octet in
> the dotted decimal representation of the IP address, and that decisions
> could be made using that at the originating end or anywhere in the 
> middle.  I argued and still do that it is mostly a useless datum and can
> not be known anywhere except on the leg between the destination station
> ant the router talking to it.
> 
> In the ARPANet discussion, what does "subnet" refer to?  It seems like
> there must have been a bigger network that I don't know anything
> about.

Well, I don't think there was any 'subnet'ing with the ARPAnet -- just 
IMP numbers, mostly.  In the IP/Internet era, as I understand it, a 
subnet just consists of a block of a power of 2 IP addresses, always 
accompanied by *some* indication, implicit or explicit of the size of the 
block[*].  [and so, for example, we allocate subnets-of-8 to many of our 
customers.].  The standard "class C" has 256 IP addrs, class B 65K[**], 
etc.  I can see a context in which the "third octet" might make sense: if 
you have a class-B allocated and you, in turn, allocate class C's to 
other parties, then the class-B holder might well think of the third 
octet as the "subnet number"

    [*] for example you will often see folk talk about /28 subnets and
    /20 subnets and such, where that's the number of bits in the 'base'
    IP addr, and so the subnet consists of 2^(32-<thatnumber>) IP
    addresses.  We've occasionally used the terminology "/32" for "a
    single IP address". 

    [**] Yes, I know generally that that either one or two of the IP
    addresses in the block isn't available..:o) 

/Bernie\

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Bernie Cosell                     Fantasy Farm Fibers
mailto:bernie at fantasyfarm.com     Pearisburg, VA
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