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dob Apprentice
Joined: 04 Oct 2002 Posts: 199 Location: S.L.P.
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Posted: Tue Feb 25, 2003 1:12 am Post subject: Ip ranges notations |
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Daemons firewalls etc all seem to use a common notation to name ip ranges.
like 3.2.1.0/32
However, since I use linux for fun and never had tcp/ip classes at school, I don't understand that notation.
It seems that 1.0.0.0/8 equals the ip range from 1.0.0.0 to 1.255.255.255
But how about /12 /16 etc.? Could someone explain me in simple terms how it works?
Thanks |
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fbleagh Tux's lil' helper
Joined: 13 Jun 2002 Posts: 98
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Posted: Tue Feb 25, 2003 1:43 am Post subject: |
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The /8 /16 /24 notation is just a shorthand version of the old datted quad system
without going into the whys and wherefores here is a basic conversion table
255.0.0.0 = /8
255.255.0.0 = /16
255.255.255.0 = /24
if you want a bit more of an explanation then read on.
255.0.0.0 in binary = 11111111 00000000 00000000 00000000
funnily enough 255 in binary is 11111111
and 255.255.0.0 in binary = 11111111 11111111 00000000 00000000
getting the idea ?
The new notation just says how many bits are being masked.
if you look at the above u can see that 255.0.0.0 is masking 8bits.
and 255.255.255.0 is masking 16 bits
I hope this is of some help.
Cheers from Down Under
Fbleagh |
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dob Apprentice
Joined: 04 Oct 2002 Posts: 199 Location: S.L.P.
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Posted: Tue Feb 25, 2003 2:26 am Post subject: |
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yep this sure helps, thanks
However, I'm not sure I fully understand how it works just yet.
1.0.0.0/12 = 1.0.0.0 -> 1.15.255.255 (since 11110000 = 240)
If that's correct, now how do I proceed the other way around?
For ex what would be the notation for 192.168.1.13 -> 192.168.1.47 |
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fbleagh Tux's lil' helper
Joined: 13 Jun 2002 Posts: 98
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Posted: Tue Feb 25, 2003 4:05 am Post subject: |
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there isn't one subnet that would only do from 192.168.1.13 192.169.1.47
the closest you would get would be the 192.168.1.0 network with the subnet mask 255.255.255.192.
As subnets start at specific points along 1--255
for example
192.168.1.0 network with the subnet mask 255.255.255.192 ( /26 )
column 1 -- Network
column 2 -- first host in subnet
column 3 -- last host in subnet
column 4 -- Broadcast Address
192.168.1.0 192.168.1.1 192.168.1.62 192.168.1.63
192.168.1.64 192.168.1.65 192.168.1.126 192.168.1.127
192.168.1.128 192.168.1.129 192.168.1.190 192.168.1.191
192.168.1.192 192.168.1.193 192.168.1.254 192.168.1.255
so if i was given the address 192.168.1.13 with a subnet mask of /26
i would be able to see addresses 192.128.1.1 --> 62 without going through a router.
or if i was 192.168.1.140/26 i would be able to see 192.168.1.129-190
I may be easier for you to explain where you are tryng to get to and i'll see what i can do to step you through it |
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dob Apprentice
Joined: 04 Oct 2002 Posts: 199 Location: S.L.P.
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Posted: Thu Feb 27, 2003 6:40 pm Post subject: |
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Actually I'm just trying to understand how to tell a daemon what ip range he should accept connections from without typing all of them 1 by 1, same for allowing/blocking ip ranges with iptables.
For instance the first time I tried to setup exim, I wanted it to relay mail for the 2 computers of my internal lan, and I assumed 192.168.0.2/2 would only let 192.168.0.2 and 192.168.0.3 send mail through it Hopefully I realized it was not correct minutes later by running an open relay test before any spammer could abuse it.
If I understood what you told me correctly, I should have used 192.168.0.0/30
I feel confused by what this has to do with routers, and your 192.168.1.140/26 example
Regarding routers I assume you're talking about the calculation of the netmask to use to have several subnets within say 192.168.0.xxx
Regarding 192.168.1.140/26, here's my understanding of how you get 192.168.1.129->190.
As I'm trying to find the calculation by knowing the solution, I put my fantasy to work but I don't get the same figures
26 means we have the 1st two bits masked, ie 11000000, that makes 192 in decimal
255-192=63
Then we divide the 1-255 range in pieces of 63 ips
We get 1->63, 64->127, 128->191, 192-255
128<140<191 so 192.168.1.140/26 = 192.168.1.128-191
Where am I wrong |
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digitalnick Apprentice
Joined: 30 Jun 2002 Posts: 243 Location: Lawrence KS USA
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Posted: Thu Feb 27, 2003 7:06 pm Post subject: |
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the /8 ect ... is called CIDER notation
its used because the regular class based ranges are limiting with classless routing its easy to setup supernets and subnets
the /number stands for how many network bits there are in the address
192.168.1.1/24 is a class c address for example its subnet is 255.255.255.0 but if we want to say buy 2 class c ranges and make them into one bigger network we need to borrow bits so we could have say a /20 netmask which would be
11111111 11111111 11110000 0000000 in binary
or
255.255.240.0
that leaves you 12 bits for hosts so you could have 2^12-2 hots instead of 2^8-2 hosts
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rtn Guru
Joined: 15 Nov 2002 Posts: 427
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Posted: Thu Feb 27, 2003 9:58 pm Post subject: |
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digitalnick wrote: | the /8 ect ... is called CIDER notation |
Isn't that CIDR?
--rtn |
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dasalvagg Apprentice
Joined: 26 Jun 2002 Posts: 183 Location: NY
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Posted: Thu Feb 27, 2003 10:13 pm Post subject: |
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yep...its CIDR
Look up classless interdomain routing. You're right, you're likely to see /xx notation on routers...its right in the name. In most cases its used to allow ISP to give customers a block of continous IP address without forcing them to buy an entire class...which you cant even do anymore. Its a good idea to understand subetting and supernetting too, these all have the same princibles in mind. Breaking apart ranges or addresses to create more useable systems. Let me give you a warning...breaking down too many ip addresses to binary will hurt your head. |
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Major Nugzz n00b
Joined: 30 Jan 2003 Posts: 6 Location: Wisconsin
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digitalnick Apprentice
Joined: 30 Jun 2002 Posts: 243 Location: Lawrence KS USA
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Posted: Thu Feb 27, 2003 11:18 pm Post subject: |
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lol i spelled it wron cider heh /me smacks himself |
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