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" <deml...@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-boun...@lists.phxlinux.org
[mailto:plug-discuss-boun...@lists.phxlinux.org] On Behalf Of
parabell...@yahoo.com
Sent: Monday, August 31, 2015 3:25 PM
To: plug-discuss@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:
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[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
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