IP and on-line identification
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Written by Robert on April 12, 2008 – 1:11 pm
Network addresses were allocated by a central Internet authority (which eventually became ICANN – Internet Corporation for Assigned Names and Numbers). Once an organization had a network address, it was responsible for allocating individual machine addresses within its address space.
A two-level system of network and machine identifiers was never very practical. Administering the records of the (60 000+) machines in a Class B network would be quite onerous. Further there were typically complicating factors. An organization might employ multiple technologies – Ethernets, Token Rings or proprietary systems (many of these LAN systems are limited in the number of machines on a particular physical network), or might have its machines distributed over many sites. These complications meant that it was best to break up the address space further. Machine addresses could be administered separately for different physical networks within the organization. Data routing could be made more efficient if the IP routers could take cognizance of different subsets of IP addresses being in different physical networks and relay data only as needed. For these reasons, it became common to ‘sub-net’ a class A, B or even C network.
Sub-netting is an internal responsibility of the organization. It is achieved by changing how the company’s own routers and switches interpret the private ‘machine address’ part of an IP address. The 24, 16 or 8 bits of address space again get broken up, this time into a ‘sub-net’ address and a machine address defined relative to its sub-net. Sub-netting is achieved by the routers using different masks to select bits from an IP address. For example, a standard class B address is composed of 16 bits of network address and 16 bits of machine address. The network part can be identified by an AND operation between the address and the bit pattern 255.255.0.0:
10001000.10101010.11110001.01010101 (Address) 136.170.241.85
11111111.11111111.00000000.00000000 (Network mask) 255.255.000.000
——————————————————————–
10001000.10101010.00000000.00000000 (Network) 136.170.0.0
10001000.10101010.11110001.01010101 (Address) 136.170.241.85
00000000.00000000.11111111.11111111 (Machine mask) 0.0.255.255
——————————————————————–
00000000.00000000.11110000.01010101 (Machine) 0.0.241.85
This IP address identifies the machine as number 241.85 within the network 136.170. The ‘network’ mask could be made larger; for example, five more bits could be allocated as a sub-net mask:
10001000.10101010.11110001.01010101 (Address) 136.170.241.85
11111111.11111111.11111000.00000000 (Net&subnet mask) 255.255.248.000
——————————————————————–
10001000.10101010.11110000.00000000 (Net&subnet) 136.170.240.0
10001000.10101010.11110001.01010101 (Address) 136.170.240.85
00000000.00000000.00000111.11111111 (Machine mask) 0.0.7.255
——————————————————————–
00000000.00000000.00000001.01010101 (Machine) 0.0.1.85
The machine is now identified as ‘1.85’within the ‘30’ subnet of the 136.170.0.0 network. There are restrictive rules on sub-netting. You cannot have machine addresses that are all zero-bits or all one-bits; nor can you have sub-nets that have all zero-bits or all one-bits as their identifiers. These restrictions cut out part of the potential address space. Instead of 32 possible sub-networks being identified by a 5-bit mask, only 30 are allowed. These sub-networks cannot have 2048 machines each, but only 2046. Instead of 65,534 possible machines, a system using a five-bit subnetmask can only have 61,380 machines. If instead you used a three-bit mask, you would have a system that had six subnets, each with up to 8190 machines (for a total of 49140).
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