how do you make a new subnet mask?

Answer 1

Depends on what you are trying to do. Subnet masking is to create two or more networks.

eg: Take a Private Class B address of 172.16.x.x (x stand for numbers). The 172.16 is your network portion and x.x your host portion. The default subnet for this network is 255.255.0.0. If you create another network with the same default mask but with an address of 172.17.x.x, you have two seperate networks. You will need a router to join these two networks. Theoretically each network can host 65,525 devices but in reality each network can handle only about 1500 (if using TCP/IP).

If you have want to subnet just the 172.16.x.x, then you will have to borrow bits from the third octet. The binary of the default subnet is 11111111.11111111.00000000.00000000. The first 11111111 is the first octet and so on. To create 2 subnets, you will borrow 1 bit from the left of the third octet (called the interesting octet) of 00000000. Now you have the third octet showing 10000000. The mask is now 11111111.11111111.10000000.00000000 or in decimals 255.255.128.0. This will give you two networks of 172.16.x.0 to 172.16.x.127 and 172.16.x.128 to 172.168.x.255 each having 128 hosts but 126 usable because the first (network address) and the last (broadcast address) IP addresses are not used for hosts. The networks have to communicate with each other using a router.

The more bits you borrow from the left the more subnets you wil have but lesser hosts. If you borrow two bits, you get 4 subnets, if you borrow three you will get 8 subnets and so on. 2 to the power of number of bits borrowed. The hosts on each network will be 2 to the power of remaining bits minus two.

I could have directed you to sites but I prefer to use my mind. In the process, I refresh myself. Hope this helps. :)

if you need help with subnetting, you can contact me at [e-mail].

Answer 2

It's really very simple ... you don't!

It's your ISP [inernet Service Provider] who assigns this to you based on the default gateway they set up for you.

Ask your ISP.

Cheers

Answer 3

http://krow.net/dict/subnet.html

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

Answer 4

http://darwin916.com

Some provider offer it...