defined
VLSM (Variable Length Subnet Mask, variable length subnet mask) specifies a different one of how different parts of a network using subnetting subnet mask code. This is effective for the case of internal network segments require different subnets for different sizes.
Introduction
VLSM fact, with the IP address for the class is. Class A network number is the first stage (the first eight bits), the first two Class B addresses are network number (the first sixteen bits), the first three paragraphs of class C is a network number (before twenty-four). VLSM is acting on the basis of the IP address based on the network do lend corresponding digit number from the host number part thereof, i.e. to increase the number of bits in network ID. Median various types of networks can be used again subnetting are: A class has twenty-four can borrow, there are sixteen Class B can borrow, there are eight class C can borrow digits (can be subdivided is the host digit number. in fact it can not be borrowed, because the whole host number 0 as the network address, broadcast address as a whole, so the actual number of bits that can be borrowed at the numbers above minus 2, borrowed as a bit subnet part).
This is a mechanism for generating a network allocation subnets of different sizes, refers to a network can be configured with different masks. Greater flexibility in the development of variable length subnet mask idea is to retain a sufficient number of hosts on each subnet at the same time, the subnet further divided into smaller subnets. Without VLSM, a subnet mask only to one network. This limits the number of hosts on the number of subnets required. Further, it based VLSM bits, and type of network is based on an 8-bit groups.
In the actual engineering practice, the network can be further divided into three or more stages subnets. At the same time, be able to consider the use of all-zero and all-one subnet to save network address space. Using a LAN on a mask 27, the support 30 can each subnet hosts (2 ^ 5-2 = 30); and for the WAN connections, each connection requires only two addresses, the ideal solution is 30 mask (2 ^ 2-2 = 2), but with the same main category network mask constraints must also use the WAN between the mask 27, so that a waste of 28 addresses.
principles
VLSM actually have classes with respect to the standard in terms of the subnet mask, the bit for the IP address of the network number of the class has a portion the default number is equivalent to the length of the mask. Class A network number is the first stage (the first eight bits), the first two Class B addresses are network number (the first sixteen bits), the first three paragraphs of class C is a network number (before twenty-four). VLSM is acting on the basis of the address of the P class has on loan from their respective host number part of the network number of bits to do, i.e. to increase the number of bits of the network number, increases the length of the mask. Median various types of networks can be used again divided into: A class has twenty-four can borrow, there are sixteen Class B can borrow, there are eight class C can borrow (the number of bits that can be subdivided host number of digits. in fact it can not be borrowed, because the P address part must have a host number, and host number part of the remaining one is meaningless, and the remaining time is not on behalf of a host number that represents the number of broadcast , so the actual may borrow up to a median of host bits minus 2). This is a mechanism for generating a network allocation subnets of different sizes, refers to a network can be configured with different masks. Development of variable length subnet mask idea is to retain a sufficient number of hosts on each subnet at the same time, there is greater flexibility to a network into multiple subnets. If you do not ⅥISM, a subnet mask only to one network. This limits the number of hosts on the number of subnets required.
Basic algorithm
VLSM is a wide range of IP network into multiple small-scale IP networks for different departments or business units of a particular internal network may display different Foreign can display the same IP network. Mentioned VLSM, have to mention that the mask. As with mask IP, having a 32-bit binary, for operation with a particular IP, the IP network number is calculated. IP for the upcoming 32-bit "and operation" with the 32-bit mask, so that you can obtain the network number.
as: IP is: 21.31.233.69 network number and the mask of 255.255.255.192:
so the subnet network number 21.31.233.64.
Application examples
A company has two main divisions: Marketing and Technology, the Ministry of Technology is divided into hardware and software of the two departments. The company applied for a full Class C IP addresses: 210.31.233.0, subnet mask of 255.255.255.0. For ease of classification management, the company uses a VLSM technique, the original master is divided into four sub-network, a subnetwork considered (not considering zero and all subnets 1). Part of which had a market subnets, namely 210.31.233.64, subnet mask of 255.255.255.192, the subnet a total of 62 IP addresses available for allocation.
to give a technical part of the subnet 210.31.233.128, subnet mask 255.255.255.192, it is further divided into two two subnets. Wherein a first two sub 210.31.233.128, allocated to a division of the subnet mask 255.255.255.224 Technology - A hardware portion, the two sub-total of 30 IP addresses available for allocation. A division of the Technology - Part software had two second subnet 210.31.233.160, subnet mask of 255.255.255.224, a total of 30 of the two subnet IP addresses available for allocation.
In the actual engineering practice, the network can be further divided into three or more stages subnets. At the same time, consider using all-zero and all-one subnet to save network address space.