In computers/internet, what is a subnet mask?

Answer 1

The previous poster had it almost right. The subnet mask is an indicator of what part of the whole 32-bit IP address identifies the network, and what part identifies the device ("host"). (The "dotted decimal notation" using addresses like "168.74.24.5" is actually shorthand for a string of 32 binary bits -- in this case 1010100 01001010 00011000 00000101. A "mask" is likewise a string of 32 bits, but it's always a bunch of 1's, followed by a bunch of 0's.) The bits in the IP Address which correspond to the 1's in the mask identify the network, while the bits which correspond to the 0's identify the host.

A subnet mask is added to the default mask of a network to break a network into smaller sub networks. Networks are split up for a couple of reasons -- one is to create separate domains for separate organizations, the other is to improve network efficiency: If there are thousands of hosts on a single network, the wires are going to be very busy, and communication will be very slow. A smaller network will be much faster.

When you're configuring a computer for a network connection, you need to enter this stuff correctly so you end up on the right network -- if you don't, you probably won't get any network connectivity at all.

Actually figuring out the masks take a fair amount of binary arithmetic, but I'll try to explain a simple example.

In a straight class A address like 10.1.8.99, the first 8 bits identify the network. (Class A networks start with numbers from 0 to 127.) The mask is 255.0.0.0, which means the "10" (the first octet) identifies the network and the "1.8.99" identifies the host. (In binary, the default class A mask is 11111111 00000000 00000000 00000000 .)

Now if you're going to split up that network into sub-networks, you use a "subnet mask" which indicates how many more bits are going to be used for the network. If you want to use 4 more bits, which will split the network into 16 sub networks (2 to the 4th power), you add 4 ones to the subnet mask immediately following the class's ones. The subnet mask would be 11111111 11110000 00000000 00000000 in binary, or 255.240.0.0 .

Likewise, with a class B network (which starts with 128 through 191), the default mask is 255.255.0.0 (meaning the first 16 bits identify the network), and you would use the bits in the third (or third and fourth) octet to split it into smaller networks; and a class C network (192 through 223) has a default mask of 255.255.255.0, and you'd use the bits in the fourth octet to split it into subnets.

Answer 2

The subnet is what the computer uses to figure out a network address. Subnets are far more complicated than just to say that class A addresses have a subnet of 255.0.0.0 and class b addresses have a subnet of 255.255.0.0 and that class c addresses have a subnet of 255.255.255.0 ( and you don't determine the class by the subnet you determine the subnet by the class) the classes are determined by the first octet of the ip address. If it is 1 - 127 (like 127.0.0.0) it is class A if it 128-191 it is class b
and if it is 192-223 class c
127.0.0.1 is reserved for the loopback.


Now to complicate things I will talk about subnetting. This is a way to use the subnet mask to break up a network to use one IP and break it up into many networks.

If I had the class C network address of 192.168.15.0 (which is a private address)
And I wanted to say break it up into 4 networks I would use the formula 2^n =>4 ( if I were using a cisco router)
for anyother router I would use 2^n-2 => 4

You then represent n as the number of bits it would take to make the formula true. ( which in this case is 2 because 2 squared is 4)
then you add the value of the bits which in binary is
110000000 or 190 in decimal
now your subnet mask becomes 255.255.255.190
(as you can see this subnet mask is different form the default for a class c address)

and this would devide your Class c address into 4 different network addresses. It gets more complicated than that when you have to account for the broadcast IP and figure out valid ip ranges and also if you were to use a different class of IP addresses. But you get the picture of what a subnet is for and what subnetting is.

Answer 3

I will try to make this as simple as possible. The subnet mask determines whether the IP address is A, B, C, etc. It will be a number, something like 255.255.255.0 Each computer on the internet has to have 3 things: IP address, Subnet Mask, Default Gateway. To find yours, open a command prompt, and type "ipconfig /all" without quotes, and hit enter.

Answer 4

A way the IP protocol figures out the destination of a packet within the network, basically its a representation of the IP address.

Answer 5

Here is a definition and examples:
http://www.webopedia.com/TERM/S/subnet_mask.html