IP range notation

Has anyone got any idiot-proof links that describe notation you may come
across for IP ranges ? ie. 201.255.0/17

Thanks :-)

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On Thu, 13 Jan 2005 23:06:15 -0000, Colin Wilson <(E-Mail Removed)>
wrote:

>Has anyone got any idiot-proof links that describe notation you may come
>across for IP ranges ? ie. 201.255.0/17

A quick google on "IP class CIDR notation" turns up (amongst many):

http://www.tcpipguide.com/free/t_IPS...gCIDRHiera.htm

http://compnetworking.about.com/od/w.../aa021003a.htm


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> >Has anyone got any idiot-proof links that describe notation you may come
> >across for IP ranges ? ie. 201.255.0/17
> A quick google on "IP class CIDR notation" turns up (amongst many):

Thanks - i`d already tried "ip range allocation" & "ip block allocation"

I hadn`t heard of CIDR before...

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> >Has anyone got any idiot-proof links that describe notation you may come
> >across for IP ranges ? ie. 201.255.0/17

I wonder... in the example I give above then, is the effective IP range
201.255.0.0 -> 201.255.255.128 or thereabouts ?

Thanks :-}

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Colin Wilson <(E-Mail Removed)> wrote:

> Has anyone got any idiot-proof links that describe notation you may come
> across for IP ranges ? ie. 201.255.0/17

This will make the most sense if you also read a little about how IP
Routing works, but a quick run through address notation could be like
this..


An IPv4 address is a 32 bit binary number. It's broken into 4 bytes, most
significant first, for writing in "dotted decimal" format, such as
212.23.10.115

Where you want to describe a range of addresses which begin and end on
"bit boundaries" (and as such they are actually routable as a single
entity), you only need to specify how many of the bits represent the
network (the remainder being used to address devices on the network).

That can be done easily with a network mask or a prefix length..

A network mask is also written in dotted decimal, for eg 255.255.255.0

The Network Address is the bitwise AND of the Address and the Network
Mask, so..

...for an address 212.23.10.115 with network mask 255.255.255.248, we
have..

212 AND 255 .. which is .. 11010100 & 11111111 .. which = 212
23 AND 255 .. which is .. 00010111 & 11111111 .. which = 23
10 AND 255 .. which is .. 00001010 & 11111111 .. which = 10
115 AND 248 .. which is .. 01110011 & 11111000 .. which = 112

...so that's on network 212.23.10.112

These can be written as..

212.23.10.115/255.255.255.248 describing the host and network
212.23.10.112/255.255.255.248 describing the network only

You can also see that there are 29 1's in the mask before the 0's. That's
the prefix length, so that can be written as..

212.23.10.115/29 describing the host and network
212.23.10.112/29 describing the network only

Both of these notations represent the addresses 212.23.10.112 through to
212.23.10.119 inclusive, although the first example in each case is
specifically representing the address 212.23.10.115.

To determine how many addresses are in a given range, simply consider the
number of 0's at the end of the (binary) network mask..

In the above example, 3 zeros, so 8 possible addresses can be
represented.

If the network mask is written in dotted decimal, there's a quick trick
for the simple cases .. For the above example, 256 - 248 is 8.

Also note that 32 (bits in an IPv4 address) - 29 (prefix length) = 3
...and that 2^3 (2 to the power of 3) is 8.

If you want to represent a range of IP addresses which is not a power of
2, then you should simply show the beginning and end, eg, the 5 addresses
in the range "212.23.10.113 - 212.23.10.117".

Sometimes people will talk about Classes of addresses. Before routing on
the Internet supported arbitrary prefix lengths (ie, anything from /0 to
/32) as it does today, the 32 bit IPv4 address space was sliced up into 6
classes of address, called Classes A, B, C, D, E, and F.

One popular misconception is that "Class C" means "/24" (in prefix length
notation). That's actually incorrect - Class C networks are /24 networks,
but not all /24 networks are Class C.

In the old Classful system, it's the most significant few bits of an
address which tell you what class the address is.

You shouldn't need to know about classful addressing now, unless you're
working with very old software/devices, multicasting, or have some reason
to work with experimental addresses - but do be aware that the terminology
is still often used, misused, and abused.

d.

Colin Wilson <(E-Mail Removed)> wrote:
>> >Has anyone got any idiot-proof links that describe notation you may come
>> >across for IP ranges ? ie. 201.255.0/17

> I wonder... in the example I give above then, is the effective IP range
> 201.255.0.0 -> 201.255.255.128 or thereabouts ?

it's 201.255.0.0 - 201.255.127.255


In the example you gave "201.255.0", so the 4th octect is assumed to be
zero (ie, 201.255.0.0).

The prefix length is 17, so the network mask is 255.255.128.0

The opposite of the network mask (sometimes called the wildcard, or host,
bits) is 0.0.127.255.

The sum of the network and the "all-ones" host bits is the final address
in the range.

Hence the range is 201.255.0.0 (the network address, with all host bits
set to zero) to 201.255.127.255 (the network address, with all host bits
set to one).

d.

On Thu, 13 Jan 2005 23:58:20 -0000, Colin Wilson <(E-Mail Removed)>
wrote:

>> >Has anyone got any idiot-proof links that describe notation you may come
>> >across for IP ranges ? ie. 201.255.0/17
>
>I wonder... in the example I give above then, is the effective IP range
>201.255.0.0 -> 201.255.255.128 or thereabouts ?
>
>Thanks :-}

There are a total of 32 bits in an IP address, broken down into 4
groups of 8., then represented as a decimal for each of those groups.

The 17 means the first 17 of those bits must be the same. Which is all
of the first two groups, and the first bit of the third group. so the
third group can be anything from 00000000 to 01111111 in binary (0 to
127 in decimal)

So your range is 201.255.0.0 -> 201.255.127.255 (you had the last two
octets the wrong way around, and forgot that the zero is one of the
128 possibilities).

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> > 201.255.0.0 -> 201.255.255.128 or thereabouts ?
> Hence the range is 201.255.0.0 (the network address, with all host bits
> set to zero) to 201.255.127.255 (the network address, with all host bits
> set to one).

I think I follow you - I just got the order wrong, and dropped a bollock
somewhere on the maths - most significant byte in the wrong order or
something :-}

I think i`ll have to wait until my head stops spinning before I try to
figure out how the network mask works, but I think I see what you`ve done
for the wildcard (255-networkmaskoctet)

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> > Has anyone got any idiot-proof links that describe notation you may come
> > across for IP ranges ? ie. 201.255.0/17
> This will make the most sense if you also read a little about how IP
> Routing works, but a quick run through address notation could be like
> this..

Thanks for that - I think its starting to gel a little now :-}

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"Colin Wilson" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) t...
> > > Has anyone got any idiot-proof links that describe notation you may
come
> > > across for IP ranges ? ie. 201.255.0/17
> > This will make the most sense if you also read a little about how IP
> > Routing works, but a quick run through address notation could be like
> > this..
>
> Thanks for that - I think its starting to gel a little now :-}

Are you saying you're ready for the exam now ? ;o)