subnet/mask

[SomeoneE1se]

Ok I may be the only one who doesn't get this but I've been up and down google and I can't for the life of me understand WTF a subnet mask is used for

I've already looked at

http://computer.howstuffworks.com/question549.htm

http://computer.howstuffworks.com/lan-switch.htm

http://en.wikipedia.org/wiki/Subnetwork

http://www.3com.com/other/pdfs/infra/corpi...n_US/501302.pdf

http://articles.techrepublic.com.com/5100-....html?tag=fdpop

http://www.cisco.com/warp/public/701/3.html#ustand_subnet

I know I've got to be missing something.

[Sparda]

It tells the computers on a network the scope of the network.

For example, if your mask is 255.255.255.0 and your computers IP address is 192.168.1.4. Your computer knows that 192.168.1.1 to 192.168.1.254 are valid IP address on the network.

Another example would be: If your mask is 255.255.0.0 and your computers IP address is 10.0.3.6, your computer knows that the valid range of IP addresses is 10.0.0.1 to 10.0.255.254.

The major exception to this is the Internet. There is effectively no mask since the whole range from 0.0.0.1 to 255.255.255.254 can be used, minus the bogon space of course.

[Iain]

I may as well jump in with another question relating to subnet masks. I know about the 3 main (5 in total) classes of networks and the SNMs 255.0.0.0, 255.255.0.0 and 255.255.255.0 but how do things work out if someone decides not to use one of these standard SNMs? Suppose I choose to use 255.132.166.0 as a SNM in conjunction with a private IP address of 192.168.0.4? What would that particular SNM tell me about my network and the IP range that's available to me? Is it still 192.168.0.1 to 192.168.0.254 inclusive?

[psychoaliendog]

Well to put in the simplest terms the Subnet mask tells you how many computers can be on that network, minus a few for routers, the gateway, etc.

eg. 11111111.11111111.11110000.00000000 is 255.255.240.0 which means that you could have 4096 unique addresses.

I believe Iain that Subnet masks are supposed to be contiguous 1's and 0's. Its a bitmask so theoretically you could have something nonstandard, but it keeps things simple. and its really not necessary, 16 bits of 0's will give you just as many IP's no matter where they are located in the mask, unless you are bent on having specific IP address.

[Iain]

Well to put in the simplest terms the Subnet mask tells you how many computers can be on that network, minus a few for routers, the gateway, etc.

Thank you. So, if I have a class C network and have my SNM as 255.255.255.0, I can have 254 hosts. If I change the SNM to 255.255.254.0, does that mean that I can have (2 * 256) - 2 = 510 hosts or some other number? I know that X.Y.Z.0 and X.Y.Z.255 are reserved IP addresses.

[SomeoneE1se]

It tells the computers on a network the scope of the network.

For example, if your mask is 255.255.255.0 and your computers IP address is 192.168.1.4. Your computer knows that 192.168.1.1 to 192.168.1.254 are valid IP address on the network.

Another example would be: If your mask is 255.255.0.0 and your computers IP address is 10.0.3.6, your computer knows that the valid range of IP addresses is 10.0.0.1 to 10.0.255.254.

The major exception to this is the Internet. There is effectively no mask since the whole range from 0.0.0.1 to 255.255.255.254 can be used, minus the bogon space of course.

This is what I thought at the start but once I started reading about it I became confused. Thanks Sparta

[Deveant]

subnet'ing does go deeper than this, specialy when your working with larger networks with labs, and sections and such, then with this u can divide rooms into having say the first 30 ips, then the next room the next 30 ips, and so on, though all sections need atleast 2 ips.

All the subnet class's are broken up with the pattern of:

Class: First String Binary:

A 0

B 10

C 110

D 1110

Also when it comes to the internet, there are actualy a lot of ristrictions, such as Class A, is restricted for research purposes.

[manuel]

http://www.solarwinds.net/products/freetoo...aspx#SubNetCalc is a great windows based subnet calculator.

Advanced IP Address Calculator v1.1 from http://www.radmin.com/download/ipcalc11.exe is another one.

I took CCNA prep classes so I know how to do it by hand, but when there are tools freely available I use these when I want to know something quick and easy.

Yes you can use non-standard subnets on private networks. I use 255.255.254.0 as a subnet to one network to allow up to 510 hosts, since you can't use 0 and 255 on both networks.

so... in my setup we use 192.168.2.0 with a subnet of 255.255.254.0, which gives us the network range of 192.168.2.0-192.168.3.255 .

Keep in mind that non-standard subnet masks are not always supported. Linksys wrt routers are an example... they simply don't work well with that network scheme.

[Iain]

Yes you can use non-standard subnets on private networks. I use 255.255.254.0 as a subnet to one network to allow up to 510 hosts, since you can't use 0 and 255 on both networks.

Shouldn't that be 508 hosts?

so... in my setup we use 192.168.2.0 with a subnet of 255.255.254.0, which gives us the network range of 192.168.2.0-192.168.3.255 .

Perfect - it's crystallising for me now! Thank you.

[manuel]

Yes you can use non-standard subnets on private networks. I use 255.255.254.0 as a subnet to one network to allow up to 510 hosts, since you can't use 0 and 255 on both networks.

Shouldn't that be 508 hosts?

so... in my setup we use 192.168.2.0 with a subnet of 255.255.254.0, which gives us the network range of 192.168.2.0-192.168.3.255 .

Perfect - it's crystallising for me now! Thank you.

Thanks for catching my error :) I knew when I posted I would make an error due to being overly tired.... :)