RFC 950 defines the use of a subnet mask which is also called as an address mask as a 32-bit value that is used to distinguish the network ID of the host ID in an IP address. The bits of the subnet mask is defined, are as follows:
- All bits are intended to be used by the network ID is set to a value of 1.
- All bits are intended to be used by the host ID is set to a value of 0.
Every host on a network that uses TCP / IP requires a subnet mask despite being on a network with one segment only. Whether it's the default subnet mask (which is used when using class-based network ID) or a customized subnet mask (which is used when making a subnet or supernet) must be configured in each node TCP / IP.
Representation of Subnet Mask
There are two methods that can be used to represent the subnet mask, namely:
Dotted Decimal Notation
The network prefix length notation
Dotted Decimal
A subnet mask is usually expressed in dotted decimal notation (dotted decimal notation), as well as the IP address. After all bits are set as the network ID and host ID, the 32-bit value will be converted to dotted decimal notation. It should be noted, that although it is represented as a dotted decimal notation, the subnet mask is not an IP address.
The default subnet mask is based on IP address classes and used in the TCP / IP network that is not divided into several subnets. The table below mentions some of the default subnet mask by using dotted decimal notation. The format is:
<alamat www.xxx.yyy.zzz> IP, mask <subnet www.xxx.yyy.zzz>
Classes address Subnet mask (binary) subnet mask (decimal)
Class A 255.0.0.0 11111111.00000000.00000000.00000000
Class B 255.255.0.0 11111111.11111111.00000000.00000000
Class C 255.255.255.0 11111111.11111111.11111111.00000000
Keep in mind, that the value above the default subnet mask can be customized by the network administrator, during a process of distribution networks (subnetting or supernetting). For example, 138.96.58.0 address is an identifier of a class B network that has been divided into several subnets by using 8-bit numbers. Eight bits are used as the host ID will be used to display the network ID that has been divided into subnets. Subnet used is a total of 24 bits (255.255.255.0) which can be used to define a custom network ID. Network identifier that has been such a subnet and the subnet mask used will then be displayed using the following notation:
138.96.58.0, 255.255.255.0
Representation of prefix length (prefix length) of a subnet mask
Because the network identifier bits must always be chosen in a form from the adjacent high-order bits, then there is a means used to represent a subnet mask by using the bits that define the network ID as a network prefix using network prefix notation as listed in the table below. Notation network prefix notation also known as Classless Inter-Domain Routing (CIDR) which is defined in RFC 1519. The format is as follows:
/ <number Bits are used as network identifier>
Classes address Subnet mask (binary) subnet mask (decimal) Length Prefix
Class A 255.0.0.0 11111111.00000000.00000000.00000000 / 8
Class B 255.255.0.0 11111111.11111111.00000000.00000000 / 16
Class C 255.255.255.0 11111111.11111111.11111111.00000000 / 24
For example, a class B network ID of 138.96.0.0 with a subnet mask of 255.255.0.0 can be represented in prefix length notation as 138.96.0.0/16.
Because all hosts within the same network using the same network ID, then all hosts within the same network must use the same network ID as defined by the subnet mask the same. For example, the notation is not the same as the notation 138.23.0.0/16 138.23.0.0/24, and both networks are not in the same address space. Network ID 138.23.0.0/16 has a range of valid IP addresses ranging from 138.23.0.1 to 138.23.255.254, while network ID 138.23.0.0/24 only have a range of valid IP addresses ranging from 138.23.0.1 to 138.23.0.254.
Determine the address of the Network Identifier
To determine the network ID of an IP address by using a subnet mask, can be done using a mathematical operation, namely by using a comparison of AND logic operation (AND comparison). In an AND comparison, the value of the two things that will compare the two is true only when the item is true, and to false if one of them false. By applying this principle into bits, a value of 1 will be obtained if the two bits that are comparable to 1, and the value 0 if there is one among the comparable value is 0.
This will perform a comparison with the logical AND operation using 32-bit IP address and a 32-bit subnet mask, known as the bitwise logical AND operation of comparison. The results of bitwise operations to subnet mask IP address that is called a network ID.
Example:
IP address 10000011 01101011 10100100 00011010 (131 107 164 026)
Subnet Mask 11111111 11111111 11110000 00000000 (255 255 240 000)
-------------------------------------------------- ----------------
Network ID 10000011 01101011 10100000 00000000 (131 107 160 000)
Table Creation subnet
subnetmask is called the classes are grouped known three classyaitu:
1. Class A, is all the IP numbers that have a subnetmask 255.0.0.0
2. Class B, is all the IP numbers that have a subnetmask 255.255.0.0
3. Class C, is all the IP numbers that have a subnetmask 255.255.255.0
Netmask combination of IP and this is the computer addressing is used. Both of these can not escape. So the writing is usually as follows: IP: 202.95.151.129 Netmask: 255.255.255.0
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