IP Subnets

Due to the explosive growth of the Internet, the principle of assigned IP addresses became too inflexible to allow easy changes to local network configurations. Those changes might occur when:

. A new type of physical network is installed at a location.

. Growth of the number of hosts requires splitting the local network into two or more separate networks.

. Growing distances require splitting a network into smaller networks, with gateways between them.

To avoid having to request additional IP network addresses in these cases, the concept of subnets was introduced. The assignment of subnets can be done locally, as the whole network still appears to be one IP network to the outside world.

The host number part of the IP address is subdivided again into a network number and a host number. This second network is termed a  subnetwork or  subnet. The main network now consists of a number of subnets and the IP address is interpreted as:

<network number><subnet number><host number>

The combination of the subnet number and the host number is often termed the local address or the  local part. Subnetting is implemented in a way that is transparent to remote networks. A host within a network that has subnets is aware of the subnetting but a host in a different network is not; it still regards the local part

of the IP address as a host number.

The division of the local part of the IP address into subnet number and host number parts can be chosen freely by the local administrator; any bits in the local part can be used to form the subnet. The division is done using a  subnet mask which is a 32 bit number. Zero bits in the subnet mask indicate bit positions

ascribed to the host number, and ones indicate bit positions ascribed to the subnet number. The bit positions in the subnet mask belonging to the network number are set to ones but are not used. Subnet masks are usually written in dotted decimal form, like IP addresses.

The special treatment of all bits zero and all bits one applies to each of the three parts of a subnetted IP address just as it does to both parts of an IP address that has not been subnetted (see 2.1.1.2, “Special IP Addresses” on page 30). For example, a subnetted Class B network, which has a 16-bit local part, could use one of the following schemes:

. The first byte is the subnet number; the second byte is the host number. This gives us 2 8 -2 (254 with the values 0 and 255 being reserved) possible subnets, each having up to 2 8 -2 (254) hosts. The subnet mask is 255.255.255.0.

. The first 12 bits are used for the subnet number and the last four for the host number. This gives us 2 12 -2 (4094) possible subnets but only 2 4 -2 (14) hosts per subnet. The subnet mask is 255.255.255.240.

There are many other possibilities. In fact, the number of subnets and hosts and future requirements should be taken into consideration before defining a subnet. In the above example, for a subnetted Class B network, there are 16 bits left for the subnet number and the host number fields. The administrator

has the choice of defining either a larger number of subnets with a small number of hosts in each, or a smaller number of subnets with many hosts. While the administrator is completely free to assign the subnet part of the local address in any legal fashion, the objective is to assign a  number of bits to the subnet number and the remainder to the local address. Therefore, it is normal to use a contiguous block of bits at the beginning of the local address part for the subnet number because this makes the addresses more readable. (This is particularly true when the subnet occupies 8 or 16 bits.) With this approach, either of the subnet masks above are “good” masks, but masks such as 255.255.252.252 and 255.255.255.15 are not. (In fact, hardly any TCP/IP implementation supports non-contiguous subnet masks, and their use is commonly discouraged, especially in CIDR environments that would become non-functional by choosing non-conventional subnet masks or network prefixes.