Subnetwork

As subnet a subnet of a network with the Internet Protocol ( IP) is called. It combines multiple successive IP addresses using a subnet mask (in the case of IPv6 is called the prefix ) to binary boundaries under a common front, the prefix together. A scheme for subdividing networks was introduced in 1985 and 1992 for the first time standardized in RFC 950, 1993, he was the method used today called Classless Inter- Domain Routing. In administratively independent areas, so-called autonomous systems, always one or more subnets are managed, which in turn can be subdivided into smaller subnets.

Representation

For a detailed presentation, see: netmask.

The separation of the subnet range is detected by means of bitwise mask of a certain part of the IP address with the subnet mask. This is obtained from an arbitrary address for the subnet containing the address belongs to the assuming this mask. If you want to designate a specific subnet, are you the first address in that subnet with the subnet mask or prefix length to. Since the mask is cumbersome notation for IPv4 with four decimals little compact and in use, one notes instead becoming more common as the number of ones prefix length, a representation that has prevailed for IPv6 from the start.

Subnets as a representation of IP networks are used in routing tables and filter definitions in both routing protocols as well as in packet filters. By routing between subnets, the Internet is structured.

Net classes

Before the introduction of CIDR ( Classless Inter- Domain Routing ) certain subnet masks were for certain areas of the IPv4 address space defined according to the first three bits of the IPv4 address (see grid class). So there were, for example, Class A addresses range from 0.0.0.0 to 127 255 255 255. This had as a mask 255.0.0.0. Other solid masks were / 16 ( Class B, mask 255.255.0.0 ) and / 24 ( class C mask 255.255.255.0). The former network classes are also the reason why there are three different private IPv4 address ranges, namely in each of the classes mentioned one.

CIDR

For a detailed presentation, see: CIDR.

CIDR overturned this fixed assignment of subnet masks for IP ranges. Instead of the predetermined fixed Masks now be used with a certain length. These consist of a number of contiguous binary ones, followed by binary zeros to 32 bits to fill in the case of IPv4 and IPv6 128-bit. Thus, there are 33 possible prefix lengths for IPv4 and 129 for IPv6.

If one shifts the boundary between solid (mask ones ) and variable (mask zeros) part can now be sub - subnets generated from a subnet. Due to the fact that the fixed part is extended by a few points, one can distinguish several sub - subnets about this " newfound " fixed locations. Since the variable range so that is automatically smaller, these subnets but are smaller.

Usage on network segments

A common use of subnets is to assign an address range to a network segment. Computers on this network segment are thereby assigned addresses from the range of each subnet.

For historical reasons, two of the addresses are of limited usefulness. These are:

• The first IP address in the subnet ( host portion is all zeros ) - this address is the network address of the subnet. This address is used by Windows 9x as a broadcast address. If there are no Windows 9x computer (Win 95, Win 98, Win ME) are on the network segment, this address can be used freely.
• The last IP address in the subnet by default also be used as a broadcast address. However, many systems now support the use of a network without broadcast address. If all systems on the network segment support this, it can be anywhere deactivated and then used normally.

The computer on the network segment use the subnet specification to determine whether there is a specific IP address in the same segment. Here, the test described earlier in this article is performed. Depending on the result, then tried to reach the address locally ( using ARP ) or remotely ( via a router ).

Utilities

For calculating network areas from given addresses and prefix lengths in different notation are freely available assistance programs. ipcalc do this for IPv4, sipcalc works for IPv4 and IPv6.