Term of the Day: Subnet Mask

[Keefe]

A mask used to determine what subnet an IP address belongs to. An IP address has two components, the network address and the host address. For example, consider the IP address 150.215.017.009. Assuming this is part of a Class B network, the first two numbers (150.215) represent the Class B network address, and the second two numbers (017.009) identify a particular host on this network.

Subnetting enables the network administrator to further divide the host part of the address into two or more subnets. In this case, a part of the host address is reserved to identify the particular subnet. This is easier to see if we show the IP address in binary format. The full address is:

10010110.11010111.00010001.00001001

The Class B network part is:

10010110.11010111

and the host address is

00010001.00001001

If this network is divided into 14 subnets, however, then the first 4 bits of the host address (0001) are reserved for identifying the subnet.

The subnet mask is the network address plus the bits reserved for identifying the subnetwork. (By convention, the bits for the network address are all set to 1, though it would also work if the bits were set exactly as in the network address.) In this case, therefore, the subnet mask would be 11111111.11111111.11110000.00000000. It's called a mask because it can be used to identify the subnet to which an IP address belongs by performing a bitwise AND operation on the mask and the IP address. The result is the subnetwork address: Subnet Mask 255.255.240.000  11111111.11111111.11110000.00000000
IP Address 150.215.017.009  10010110.11010111.00010001.00001001
Subnet Address 150.215.016.000  10010110.11010111.00010000.00000000


The subnet address, therefore, is 150.215.016.000.

Ok that is good how about VLSM and CIDR?I am trying to learn these two but the vlsm is kinda confusing.

[Keefe]

I can create a write up about CIDR...but I do not even know what VLSM is!?

VLSM= variable length subnet mask supposedly TCP/IP version6 It is used in W2K and I am trying to learn it but it is confusing. With it you can borrow bits from an existing subnet and subnet the subnet. This way if you needed 40 subnets, if you borrowed enough to get 32 addresses,you could subnet the subnets to make up the other 8 needed. Instead of having to borrow enough to make 64 and waste the other 24.

[Keefe]

How can you assigne a subnet mask like that in windows 2k though?

Quite simple my dear admin - you download the IPV6 patches for windows 2000 along with the proper scripts and woot, your ready to rock and roll. In *nix you need the latest net-tools package to get in on the goodness of ipv6. I have played quake over an ipv6 network w00t!