:: TCP/IP Subnetting ::

IP Tutorial: Subnetting

Subnets allow network traffic between hosts to be segregated based on the network's configuration. In IP networking, traffic takes the form of packets. IP subnets improve network security and performance to some degree by organizing hosts into logical groups.

Subnet Masks
Probably the most easily recognizable aspect of subnetting is the "mask." Just like IP addresses, subnet masks contain four bytes (32 bits) and usually appear in the same "dotted decimal" notation. For example, a very common subnet mask in its binary representation

11111111 11111111 11111111 00000000

will usually be shown in the equivalent, more human-readable form
255.255.255.0

Masking Rules
A subnet mask neither serves as an IP address nor does it exist independently from them. Instead, subnet masks must be applied to IP addresses. Masking a full IP address has the effect of splitting it into two parts -- an "extended network address" and a host address.

For a subnet mask to be valid, its leftmost bits must be set to one; a mask of all zeros

00000000 00000000 00000000 00000000

is invalid. In addition, its rightmost bits must be set to zero; the mask of all ones
11111111 11111111 11111111 11111111

is likewise invalid. In other words, all valid subnet masks contain two parts: the all-ones left side (the extended network portion) and the all-zeros right side (the host portion).
Subnetting in Practice
An extended network address includes the basic network address as well as additional bits that represent the "subnet number." Used in conjuction with a network address, a subnet number supports a two-level, "extended" addressing scheme recognized in a standard way by implementations of IP. Taken together, the extended network address with the host address actually produce a three-level scheme.

Consider the following real-world example. A small business plans to use the "192.168.1.0" network for its internal (intranet) hosts. The human resources department wants their computers to be on a controlled part of this network because they store payroll information and other sensitive employee data. But because this is a Class C network, its default subnet mask of "255.255.255.0" will allow all computers to be peers on the network by default.

The first four bits of 192.168.1.0 -- 1100 -- place this network in the Class C range and also fix the length of the network address at 24 bits. To subnet this network, more than 24 bits must be set to one on the left side of the subnet mask. For instance, the 25-bit mask "255.255.255.128" creates a two-subnet network as follows.

Network address (24 bits) Subnet number (1 bit) Extended network Host address range
11000000 10101000 00000001 0 192.168.1.0 192.168.1.1 - 192.168.1.127
11000000 10101000 00000001 1 192.168.1.128 192.168.1.129 - 192.168.1.255

For every additional bit set to one in the mask, another bit becomes available in the subnet number to index additional subnets. A two-bit subnet number can support up to four subnets, a three-bit number supports up to eight, and so on.

Private Networks
The governing bodies that administer the Internet Protocol have identified certain networks as reserved for internal use. In general, intranets that use these networks can reduce the difficulty in administering their IP configuration and Internet access. These three networks, along with their default masks, are listed below.
Network address Default mask
10.0.0.0 255.0.0.0
172.16.0.0 255.240.0.0
192.168.0.0 255.255.0.0

Consult RFC 1918 for more details about these special networks.

Conclusion
Subnetting allows network administrators some flexibility in defining relationships among network hosts. Hosts on different subnets can only "talk" to each other through specialized network gateway devices like routers. The ability to filter traffic between subnets can make more bandwidth available to applications and can limit access in desirable ways.