Network Masks

Network masks are confusing me a wee bit.

I think I understand that 192.168.1.1/255.255.255.255 refers to the
specific address of 192.168.1.1

192.168.1.0/255.255.255.0 I think refers to the entire range from
192.168.1.0 to 192.168.1.255 so can the preceeding IP address actually
be anything in the 192.168.1 range?

Now with a filter of 255.255.255.252 I think this gives a range of 4
IP addresses, I'm thinking, if the address was given as
192.168.1.254/255.255.255.252 what addresses would be covered?

Geoff Lane

Geoff Lane wrote in message
(E-Mail Removed):

> Network masks are confusing me a wee bit.
>
> I think I understand that 192.168.1.1/255.255.255.255 refers to the
> specific address of 192.168.1.1
>
> 192.168.1.0/255.255.255.0 I think refers to the entire range from
> 192.168.1.0 to 192.168.1.255 so can the preceeding IP address actually
> be anything in the 192.168.1 range?

Yes, except 192.168.1.0 and 192.168.1.255 which are reserved for broadcast
and multicast addresses.


> Now with a filter of 255.255.255.252 I think this gives a range of 4
> IP addresses, I'm thinking, if the address was given as
> 192.168.1.254/255.255.255.252 what addresses would be covered?

I think it's convention to express a subnet by referring to the lowest value
in the range, rather than an arbitrary address within the range - so it
would normally be stated as 192.168.1.252/255.255.255.252 or
192.168.1.252/30 (30 = number of 1s in subnet mask), covering addresses 252,
253, 254 and 254. I think 252 and 255 would be reserved for broadcast,
leaving you with just two usable addresses.

On Wed, 19 Oct 2005 22:17:12 +0100, "Martin Underwood" <a@b> wrote:

>> Network masks are confusing me a wee bit.

>> Now with a filter of 255.255.255.252 I think this gives a range of 4
>> IP addresses, I'm thinking, if the address was given as
>> 192.168.1.254/255.255.255.252 what addresses would be covered?
>
>I think it's convention to express a subnet by referring to the lowest value
>in the range, rather than an arbitrary address within the range - so it
>would normally be stated as 192.168.1.252/255.255.255.252 or
>192.168.1.252/30 (30 = number of 1s in subnet mask), covering addresses 252,
>253, 254 and 254. I think 252 and 255 would be reserved for broadcast,
>leaving you with just two usable addresses.

Thanks for quick reply, I'm slowly getting there :-)))

I understand about the reserved addresses within ranges so your
explanation was helpful.

Geoff Lane

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Geoff Lane wrote:
> Network masks are confusing me a wee bit.
>
> I think I understand that 192.168.1.1/255.255.255.255 refers to the
> specific address of 192.168.1.1
>
> 192.168.1.0/255.255.255.0 I think refers to the entire range from
> 192.168.1.0 to 192.168.1.255 so can the preceeding IP address actually
> be anything in the 192.168.1 range?
>
> Now with a filter of 255.255.255.252 I think this gives a range of 4
> IP addresses, I'm thinking, if the address was given as
> 192.168.1.254/255.255.255.252 what addresses would be covered?

I've found this site useful in the past (and present):

IP Address Subnetting Tutorial, Ralph Becker
http://www.ralphb.net/IPSubnet/index.html

HTH
- --
Adam Piggott, Proprietor, Proactive Services (Computing).
http://www.proactiveservices.co.uk/

Please replace dot invalid with dot uk to email me.
Apply personally for PGP public key.
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"Martin Underwood" <a@b> wrote in message
news:4356b7db$0$6536$(E-Mail Removed)...
> Geoff Lane wrote in message
> (E-Mail Removed):
[snip]
> > 192.168.1.0/255.255.255.0 I think refers to the entire range from
> > 192.168.1.0 to 192.168.1.255 so can the preceeding IP address actually
> > be anything in the 192.168.1 range?
>
> Yes, except 192.168.1.0 and 192.168.1.255 which are reserved for
> broadcast and multicast addresses.

The lowest address in the range is called the network address. The highest
is the subnet broadcast address. As you wrote, both these addresses are
reserved (though I think the reasons are mostly/entirely historical in the
former case). Multicast is an entirely different kettle of fish; multicast
addresses are in 224.0.0.0/4.

Alex

Alex Fraser wrote in
(E-Mail Removed):

> "Martin Underwood" <a@b> wrote in message
> news:4356b7db$0$6536$(E-Mail Removed)...
>> Geoff Lane wrote in message
>> (E-Mail Removed):
> [snip]
>>> 192.168.1.0/255.255.255.0 I think refers to the entire range from
>>> 192.168.1.0 to 192.168.1.255 so can the preceeding IP address
>>> actually be anything in the 192.168.1 range?
>>
>> Yes, except 192.168.1.0 and 192.168.1.255 which are reserved for
>> broadcast and multicast addresses.
>
> The lowest address in the range is called the network address. The
> highest is the subnet broadcast address. As you wrote, both these
> addresses are reserved (though I think the reasons are
> mostly/entirely historical in the former case). Multicast is an
> entirely different kettle of fish; multicast addresses are in
> 224.0.0.0/4.

I was nearly right! I should have realised that broadcasting (to all
computers within the subnet) would use an address within the subnet but
multicasting (to all computers on the network segment, irrespective of
subnet) would use a special subnet.

In message <4356bc70$0$38044$(E-Mail Removed)>, Adam Piggott
<(E-Mail Removed)> writes
>Geoff Lane wrote:
>> Network masks are confusing me a wee bit.
>>
>> I think I understand that 192.168.1.1/255.255.255.255 refers to the
>> specific address of 192.168.1.1
>>
>> 192.168.1.0/255.255.255.0 I think refers to the entire range from
>> 192.168.1.0 to 192.168.1.255 so can the preceeding IP address actually
>> be anything in the 192.168.1 range?
>>
>> Now with a filter of 255.255.255.252 I think this gives a range of 4
>> IP addresses, I'm thinking, if the address was given as
>> 192.168.1.254/255.255.255.252 what addresses would be covered?
>
>I've found this site useful in the past (and present):
>
>IP Address Subnetting Tutorial, Ralph Becker
>http://www.ralphb.net/IPSubnet/index.html
>
>HTH
If you have broadband, you might like http://www.howtosubnet.com

I worked out subnet masks by imagining them as a ruler, you cover the
TCP/IP address with the ruler until you can't see any more of the bits
of the subnet mask and then any bits still showing of the IP address are
the network. I guess that works best if you realise the binary of the
address.
--
Clint Sharp

In message <ZrznBRBnYuVDFw+(E-Mail Removed)>, Clint Sharp
<(E-Mail Removed)> writes
>If you have broadband, you might like http://www.howtosubnet.com
>
>I worked out subnet masks by imagining them as a ruler, you cover the
>TCP/IP address with the ruler until you can't see any more of the bits
>of the subnet mask and then any bits still showing of the IP address
>are the network.
Bugger, posting at 25 to 1 in the morning is not generally conducive to
accuracy, I should have said;
and then any bits still showing of the IP address are the host range
minus the broadcast and network addresses.
>I guess that works best if you realise the binary of the address.

--
Clint Sharp

"Clint Sharp" <(E-Mail Removed)> wrote in message
news:ZrznBRBnYuVDFw+(E-Mail Removed)...
> I worked out subnet masks by imagining them as a ruler, you cover the
> TCP/IP address with the ruler until you can't see any more of the bits
> of the subnet mask and then any bits still showing of the IP address are
> the network. I guess that works best if you realise the binary of the
> address.

If you understand binary and boolean logic, then it is a small step to grasp
the idea of bitwise logical operations, and with that pretty much everything
related to subnet masks should be trivial.

Alex