Understanding IP class range.

Thu Sep 3 21:18:13 MST 2015

Thanks Kevin!!

On 2015-08-31 21:20, Kevin Fries wrote:
> I am so embarrassed, I crossed up my 1's and 0's in my original post.
> Let me correct it now.
> 
> 0... Is Class A.  This is designed for the largest network providers.
> The subnet mask is 255.0.0.0.  IP addresses in this range will start
> with a first octet less than 128.  RFC 1918 reserves the 10.x.x.x
> network as unusable on the public internet.
> 
> 10... Is Class B.  These were designed for large corps and ISPs.  The
> subnet mask is 255.255.0.0.  IP addresses in this range will start
> with a first octet between 128 and 191.  RFC 1918 reserves addresses
> 172.[16-31].x.x for private only use.
> 
> 110... Is Class C.  These were designed for most corporations and have
> a subnet mask of 255.255.255.0.  IP addresses in this range will have
> a first octet between 192 and 223.  192.168.x.x again is reserved by
> RFC 1918 for private use.
> 
> 1110... Is Class D.  This is for multi broadcast networks and there is
> never used as a primary interface.  You will often see these addresses
> used for things like NTP (Network Time Protocol) in broadcast mode.
> Addresses will have a first octet between 224 and 240.
> 
> Finally, there is 1111... Which is the "for future use" Class E, which
> never got used.
> 
> The subnet mask originally had a spec that allowed "wholes" in it.  So
> according to the original spec, 255.240.255.0 could be a legit subnet
> mask.  But the practical use for such a config never materialized, and
> most routers to this day still won't support it.  So... All the ones
> end up on the left side of the mask, and all the zeros end up on the
> right.  So a shorthand notation emerged called CIDR.  In CIDR
> notation, you simply use a slash and the number of ones.
> 
> So, 255.256.255.0 = 11111111.11111111.11111111.00000000 counting the
> ones and you get /24.
> 
> The idea of an address and subnet mask are simple.  Take your IP
> address and write it in binary.  Take your mask and write it in
> binary.  Doing a binary AND on the values gives you the Network
> address.  Not the mask and do a binary AND, and you have the host
> address.  A binary OR on the same values gives you the broadcast
> address.
> 
> Now to use all this fun info...
> 
> If Machine-A (i.e. a desktop) wants to talk to Machine-B (i.e. a web
> server) the Network addresses are compared and it they are the same,
> an ARP broadcast is used to identify that MAC address of Machine-B if
> it is not already known, so the machines can communicate directly with
> each other.  If they are on different networks, the request is
> forwarded to your default gateway to be forwarded (aka routed) to it
> destination.
> 
> Now, if we use the standard Class C address (let's say 192.168.0.x/24)
> we have 256 possible addresses.  All 0's is reserved for the Network,
> and all 1's is reserved for the Broadcast.  So 254 can be used for
> actual machines.  But what if I need more than 254 addresses?  Let's
> say I am supporting a network with 800 machines.  Now what?  Well,
> stealing one bit from the Network and giving it to the host would
> allow me 510 addresses, and 2 Bits would give me 1022.  So how do I
> steal those bits?  Simple, declare my mask as /22 or 255.255.252.0.
> With this example you can see why CIDR notation has become so
> popular... Easier math. But, at the end of the day... It all means the
> same thing... It's all about the binary notation.
> 
> Sorry for my gaff, should have tried to reply while loading the car
> with groceries, lol
> 
> Kevin
> On Aug 31, 2015 9:22 PM, "David Demland" <demland at cox.net> wrote:
> 
>> I wrote this paper to help with my students when I am teaching
>> Subnetting I
>> hope that you might find this a different way to look at
>> networking.
>> 
>> David
>> 
>> -----Original Message-----
>> From: plug-discuss-bounces at lists.phxlinux.org
>> [mailto:plug-discuss-bounces at lists.phxlinux.org] On Behalf Of
>> parabellum7 at yahoo.com
>> Sent: Monday, August 31, 2015 3:25 PM
>> To: plug-discuss at lists.phxlinux.org
>> Subject: Re: Understanding IP class range.
>> 
>>> Very nice Kenn!!
>> 
>> Thanks! Good to revisit this stuff from time to time, rattle the
>> old
>> brain-box and make sure there's still something in there! :)
>> 
>>> Ok so if I had 192.168.1.0/25 [1] the range would be 192.168.1.0
>> -
>>> 192.168.1.127
>> 
>> Yes.
>> 
>>> /26 would be 192.168.1.0 - 192.168.1.192 ? or is it 192.168.1.0
>> -
>> 192.168.1.64 ?
>> 
>> /26 would yield 192.168.1.0 - 192.168.1.63
>> 
>>> Not sure how to make a net mask out of 192.168.1.0/25 [1] -
>> would it be
>> 255.255.255.128?
>> 
>> Yes
>> 
>> The calculator Daniel provided is good:
>> 
>> http://www.subnet-calculator.com/cidr.php [2]
>> 
>> The one kitepilot provided is also nice because it illustrates the
>> full
>> binary.
>> 
>> http://jodies.de/ipcalc [3]
>> 
>> They're neet to play with and better than others I've seen. I've
>> bookmarked
>> both, thanks for sharing these resources.
>> 
>> --Kenn
>> 
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> 
> 
> Links:
> ------
> [1] http://192.168.1.0/25
> [2] http://www.subnet-calculator.com/cidr.php
> [3] http://jodies.de/ipcalc
> [4] http://lists.phxlinux.org/mailman/listinfo/plug-discuss
> [5] http://www.avg.com
> 
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-- 
Keith Smith