Subnet ?

Can someone who is up on networking tell me what the subnet is and what its
for?

My netgear DG834G has a subnet of 255.255.255.0 at present and I have been
advised by a friend recently that I should set the subnet to 255.255.255.253
as this will make the network more sucure as there will only be three
addresses available on the subnet.

What is the subnet, and is there any good reason to alter the default
settings in my netgear?

Thanks
John

"John" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> Can someone who is up on networking tell me what the subnet is and what
its
> for?
>
> My netgear DG834G has a subnet of 255.255.255.0 at present and I have been
> advised by a friend recently that I should set the subnet to
255.255.255.253
> as this will make the network more sucure as there will only be three
> addresses available on the subnet.
>
> What is the subnet, and is there any good reason to alter the default
> settings in my netgear?
>
> Thanks
> John
>
>
Briefly, a subnet is a range of valid addresses. This range is determined
by taking an IP address (e.g. 192.168.0.0) and a subnet mast (e.g.
255.255.255.0). All the valid addresses in this subnet will have the same
IP address bit set as the address logically ANDed with the subnet. So
taking the top address portion:

255 = 0xFF i.e. all bits. 0xFF AND <any byte> => <same byte>

so all addresses in your subnet have top portion 0xFF AND 0xC0 (192) => 0xC0
(192)

Similarly for all other addresses which gets you...

192.168.0.XXX where XXX can be any address from 0 to 255 because the bottom
bits of the subnet is 0x00 which means that those bits are not fixed.

Now 253 in the bottom bit is 0xFD or 11111101 in binary. This leaves only a
single bit free so in fact you can only get 2 addresses from this mask.
These will be 192.168.0.2 and 192.168.0.0.

Aside: I have a vague memory that X.Y.Z.0 may be a "special case", but I'm
not totally sure.

Now the router gets one address, your computer gets the other so providing
they're both active, that locks out anyone else. OTOH, anyone smart enough
to defeat WEP/WPA-PSK (you are using wireless with security aren't you
;-) ), can probably find a way around this, possibly by forcing your
computer to fail and then nicking the IP address assigned to it!

Paul DS.

In article <(E-Mail Removed)>, John says...
> Can someone who is up on networking tell me what the subnet is and what its
> for?
>
> My netgear DG834G has a subnet of 255.255.255.0 at present and I have been
> advised by a friend recently that I should set the subnet to 255.255.255.253
> as this will make the network more sucure as there will only be three
> addresses available on the subnet.
>
He's right but whats the point? What do you do when you want to add
more machines? You have to go off and configure the lot.

A far better way is to use MAC address filtering. If your mate is so
paranoid he's done what he suggested then TBH he needs help/

> What is the subnet, and is there any good reason to alter the default
> settings in my netgear?
>
How good is your binary maths?
Long version at:

http://www.learntosubnet.com/


--
Conor

Windows & Outlook/OE in particular, shipped with settings making them
as open to entry as a starlet in a porno. Steve B

Paul D.Smith wrote:
> "John" <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed)...
>
>>Can someone who is up on networking tell me what the subnet is and what
>
> its
>
>>for?
>>
>>My netgear DG834G has a subnet of 255.255.255.0 at present and I have been
>>advised by a friend recently that I should set the subnet to
>
> 255.255.255.253
>
>>as this will make the network more sucure as there will only be three
>>addresses available on the subnet.
>>
>>What is the subnet, and is there any good reason to alter the default
>>settings in my netgear?
>>
>>Thanks
>>John
>>
>>
>
> Briefly, a subnet is a range of valid addresses. This range is determined
> by taking an IP address (e.g. 192.168.0.0) and a subnet mast (e.g.
> 255.255.255.0). All the valid addresses in this subnet will have the same
> IP address bit set as the address logically ANDed with the subnet. So
> taking the top address portion:
>
> 255 = 0xFF i.e. all bits. 0xFF AND <any byte> => <same byte>
>
> so all addresses in your subnet have top portion 0xFF AND 0xC0 (192) => 0xC0
> (192)
>
> Similarly for all other addresses which gets you...
>
> 192.168.0.XXX where XXX can be any address from 0 to 255 because the bottom
> bits of the subnet is 0x00 which means that those bits are not fixed.
>
> Now 253 in the bottom bit is 0xFD or 11111101 in binary. This leaves only a
> single bit free so in fact you can only get 2 addresses from this mask.
> These will be 192.168.0.2 and 192.168.0.0.

If he has a subnet of 253 is this actually valid.I thought the Highest
number was 252, this gives 64 subnets consisting of:
1xnetwork ID
1xrouter IP
1xBroadcast IP
1xPC IP
To get at least 3 IP's for PC's surely he must use a subnet of
255.255.255.248
I am open to correction as I don't have a calculator.
>
> Aside: I have a vague memory that X.Y.Z.0 may be a "special case", but I'm
> not totally sure.
>
> Now the router gets one address, your computer gets the other so providing
> they're both active, that locks out anyone else. OTOH, anyone smart enough
> to defeat WEP/WPA-PSK (you are using wireless with security aren't you
> ;-) ), can probably find a way around this, possibly by forcing your
> computer to fail and then nicking the IP address assigned to it!
>
> Paul DS.
>
>

On Mon, 4 Apr 2005 14:53:53 +0100, "Paul D.Smith"
<(E-Mail Removed)> wrote:

>"John" <(E-Mail Removed)> wrote in message
>news:(E-Mail Removed)...
>> Can someone who is up on networking tell me what the subnet is and what
>its
>> for?
>>
>> My netgear DG834G has a subnet of 255.255.255.0 at present and I have been
>> advised by a friend recently that I should set the subnet to
>255.255.255.253
>> as this will make the network more sucure as there will only be three
>> addresses available on the subnet.
>>
>> What is the subnet, and is there any good reason to alter the default
>> settings in my netgear?
>>
>> Thanks
>> John
>>
>>
>Briefly, a subnet is a range of valid addresses. This range is determined
>by taking an IP address (e.g. 192.168.0.0) and a subnet mast (e.g.
>255.255.255.0). All the valid addresses in this subnet will have the same
>IP address bit set as the address logically ANDed with the subnet. So
>taking the top address portion:
>
>255 = 0xFF i.e. all bits. 0xFF AND <any byte> => <same byte>
>
>so all addresses in your subnet have top portion 0xFF AND 0xC0 (192) => 0xC0
>(192)
>
>Similarly for all other addresses which gets you...
>
>192.168.0.XXX where XXX can be any address from 0 to 255 because the bottom
>bits of the subnet is 0x00 which means that those bits are not fixed.
>
>Now 253 in the bottom bit is 0xFD or 11111101 in binary. This leaves only a
>single bit free so in fact you can only get 2 addresses from this mask.
>These will be 192.168.0.2 and 192.168.0.0.
>
>Aside: I have a vague memory that X.Y.Z.0 may be a "special case", but I'm
>not totally sure.
>
>Now the router gets one address, your computer gets the other so providing
>they're both active, that locks out anyone else. OTOH, anyone smart enough
>to defeat WEP/WPA-PSK (you are using wireless with security aren't you
>;-) ), can probably find a way around this, possibly by forcing your
>computer to fail and then nicking the IP address assigned to it!
>
>Paul DS.
>
Stictly, 255.255.255.253 gives no addresses, as "0" is the network and
"2" is the broadcast address, leaving nothing for devices. I actually
use 240 (ie 14 addresses plus network and broadcast), but for no good
reason.

If you enable the firewall in your netgear, that should disable most
incoming unknown connections and may have an option to remove the
internal (private) IP addresses from outgoing packets.
PeeGee
--
The reply address is a spam trap. If you need to reply directly, put the UK where it should be - first.

"NBT" <(E-Mail Removed)> wrote in message
news:d2rkss$45h$(E-Mail Removed)...
> If he has a subnet of [255.255.255.]253 is this actually valid.

No, it's not valid. In binary, a valid subnet mask consists of n 1s followed
by m 0s, where n >= 0, m >= 0, and n + m = 32.

> I thought the Highest number was 252, this gives 64 subnets consisting of:
> 1xnetwork ID
> 1xrouter IP
> 1xBroadcast IP
> 1xPC IP
> To get at least 3 IP's for PC's surely he must use a subnet of
> 255.255.255.248

Yep, although CIDR blurs the concept of networks and subnets.

Alex