Sub-network

The word sub-network has two significances. Its old but more general significance is a network (Data-processing network) physical being part of a more total network (). On the level of IP, a sub-network is a subset of a network of class (). The remainder of this article relates to this last significance.

The use of sub-network ( Subnetting ) in a network IP makes it possible to divide a large unit network in what seems several sub-networks. This notion was introduced before the arrival of the classes of network into IPv4, to make it possible only one Site to have a certain number of lans (Lan). Even after the introduction of the classes of network, the sub-networks remain useful to reduce the number of entries in the table of Routage for Internet by hiding information on the individual sub-networks of a site. Moreover, that made it possible to reduce the overload network (), by dividing the receiving number of hosts of the calls broadcast IP.

Mask sub-network

A mask of sub-network (also known under the name of subnet mask , netmask and address mask ) is a mask () indicating the number of Bit S used to identify the sub-network, and the number of bits characterizing the hosts (what indicates also the number of possible hosts in this sub-network).

The masks of sub-network use the same representation as those of the addresses. With IPv4, the decimal notation at point () uses four numbers going from zero to 255, separated by points (for example 255.128.0.0).

But in the masks of sub-network, only certain numbers are authorized: 0,128,192,224,240,248,252,254 and 255 (corresponding to the binary bytes of values 00000000,10000000,11000000, etc).

Another representation is possible with a number Hexadécimal of eight digits (for example, FF.80.00.00 = 255.128.0.0).

A shorter form is known under the name of Notation CIDR (acronym of Classless Inter-Domain Routing). It gives the number of the network followed by a slash (" /") and the number of bits with 1 in the binary notation (Binary system) of the mask of sub-network (i.e. the number of important bits in the number of the network). For example, 192.0.2.96 /28 indicates a Adresse IP where the first 28 bits are used as addresses network (what is identical to 255.255.255.240).

Concept of the sub-network

The IPv4 addresses are made up of three parts: the network, the sub-network (which is now regarded as making party of the network, although it formed part in the beginning of the last part), and the host. There exist 5 classes () IP addresses. the address 127.0.0.1 network is put aside because it was conceived for the loopback and cannot be affected with a network.

Example

Let us consider address IP 12.11.10.9 with a mask of 255.254.0.0 sub-network (or differently 12.11.10.9 /15):

the address host 12.11.10.9 (Decimal) => 00001100 00001011 00001010 00001001 (Binary).

the most significant bit is zero, therefore we have a network of class has with 12.0.0.0 as addresses network (see).

the mask of 255.254.0.0 sub-network (decimal) => 11111111 11111110 00000000 00000000 (binary).

the mask of sub-network extends the address network of seven additional bits (254). Blow, we have a number of sub-network of 10 and one address wide area network of 12.10.0.0 (decimal) = 00001100 00001010 00000000 00000000 (binary) (AND logic between 12.11.10.9 and 255.254.0.0).

the remaining part indicating the host is 0.1.10.9 (decimal) => 00000000 00000001 00001010 00001001 (binary).

To determine the number of hosts and sub-networks of a particular network is rather simple if you know the mask of sub-network. Let us say that you have the address 154.4.32.0 network (thus of class B thus 16 bits for the network part) with 255.255.224.0 like masks sub-network. This address network can be also written 154.4.32.0 /19

Address 154.4.32.0 network (decimal) => 10011010.00000100.00100000.00000000 (binary)

Address of 255.255.224.0 sub-network (decimal) => 11111111.11111111.11100000.00000000 (binary)

the mask of sub-network includes/understands 19 bits for the part network of the address and 13 bits for the host part.

$2^ {3} = 8$ (19 - 16 = 3) possible sub-networks available according to the RFC 1812. By using old standard RFC 950, the number of sub-networks usable is of 6. This is due to the fact that the RFC 950 (section 2.1, page 5) does not support the sub-networks with only of the 1 or only of the 0.

$2^ {13} - 2 = 8190$ possible hosts on each sub-network.

Deduct hosts and sub-networks

It is possible to determine the number of necessary sub-networks in each mask of sub-network with the number of hosts.

For example, we will use the sub-network of class B, 255.255.0.0.

The binary format is written 11111111.11111111.00000000.00000000.

By adding some bits of additional mask for our sub-network, we can determine the number of sub-networks.

11111111.11111111. 11 000000.00000000 => 255.255.192.0 (/18)

We can now discover the number of sub-networks with 2 with the power of the number of bits which we added. In our example, we added 2 of them: $2^ {2} = 4$ what gives us the number of sub-network. To obtain the real number of sub-network, we withdraw 2 of this value.

By using the bit the least significant (lsb, acronym of Least significant bit) sub-network, we discover the number of hosts. The LSB of 11111111.11111111.11000000.00000000 is 1000000.00000000 => 16384. To obtain the real number of hosts, we withdraw 2 of this value because of the need for an address for the network and an address for broadcast.

Private networks

IANA (acronym of “The Internet Assigned Numbers Authority”) held the three following blocks of addresses for the private networks (RFC1918):

Nonstandard sub-networks

Sub-network zero

Illegal although usable, it is the first sub-network. For example, the 1.0.0.0 sub-network with 255.255.0.0 like masks sub-network. The problem with this sub-network is that the address Unicast for the sub-network is the same one as the address unicast for the network of class has complete.

External bonds

Free Online subnet calculators
Online subnet calculators
IP Tutorial - Subnet Masks and Subnetting
IP address Subnetting Tutorial - Subnetting
RFC 950 Internet Subnetting Procedure
RFC 1812 New Internet Subnetting Procedure
Utility off subnets off Internet networks
IP-Subnet-Mask numbers
LearnToSubnet, has understanding Internet-movie tutorial lesson series in subnetting
RFC1101: DNS Encodings off Network Names and Other Types

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