Windows 7 / Networking

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Classless Inter-Domain Routing

At the time when IP was developed, no one imagined that the 32-bit address space would ever be exhausted. In the early 1980s, there were no networks that had 65,536 computers, never mind 16 million, and no one worried about the wastefulness of assigning IP addresses based on these classes.

Because of that wastefulness, classful addressing was gradually obsolesced by a series of subnetting methods, including variable length subnet masking (VLSM) and eventually Classless Inter-Domain Routing (CIDR). CIDR is a subnetting method that enables administrators to place the division between the network bits and the host bits anywhere in the address, not just between octets. This makes it possible to create networks of almost any size.

CIDR also introduces a new notation for network addresses. A standard dotted-decimal address representing the network is followed by a forward slash and a numeral specifying the size of the network identifying prefix. For example, 192.168.43.0/24 represents a single Class C address that uses a 24-bit network identifier, leaving the other 8 bits for up to 254 host identifiers. Each of those hosts would receive an address from 192.168.43.1 to 192.168.43.254, using the subnet mask 255.255.255.0.

However, using CIDR, an administrator can subnet this address further, by allocating some of the host bits to create subnets. To create subnets for four offices, for example, the administrator can take two of the host identifier bits, changing the network address in CIDR notation to 192.168.43.0/26. Because the network identifier is now 26 bits, the subnet masks for all four networks will now be 11111111.11111111.11111111.11000000, in binary form, or 255.255.255.192 in standard decimal form. Each of the four networks will have up to 62 hosts, using the IP address ranges shown in Table-2.

TABLE-2 Sample CIDR 192.168.43.0/26 networks

NETWORK 	STARTING IP 	ENDING IP 	SUBNET MASK
ADDRESS		ADDRESS		ADDRESS
192.168.43.0	192.168.43.1	192.168.43.62	255.255.255.192
192.168.43.64	192.168.43.65	192.168.43.126	255.255.255.192
192.168.43.128	192.168.43.129	192.168.43.190	255.255.255.192
192.168.43.192	192.168.43.193	192.168.43.254	255.255.255.192

If the administrator needs more than four subnets, changing the address to 192.168.43.0/28 adds two more bits to the network address, for a maximum of 16 subnets, each of which can support up to 14 hosts. The subnet mask for these networks would therefore be 255.255.255.240.

Public and private IPv4 addressing

For a computer to be accessible from the Internet, it must have an IP address that is both registered and unique. All of the web servers on the Internet have registered addresses, as do all of the other types of Internet servers.

The IANA is the ultimate source for all registered addresses; managed by the Internet Corporation for Assigned Names and Numbers (ICANN), this organization allocates blocks of addresses to regional Internet registries (RIR), which allocate smaller blocks in turn to Internet service providers (ISPs). An organization that wants to host a server on the Internet typically obtains a registered address from an ISP.

Registered IP addresses are not necessary for workstations that merely access resources on the Internet. If organizations used registered addresses for all of their workstations, the IPv4 address space would have been depleted long ago. Instead, organizations typically use private IP addresses for their workstations. Private IP addresses are blocks of addresses that are allocated specifically for private network use. Anyone can use these addresses without registering them, but they cannot make computers using private addresses accessible from the Internet.

The three blocks of addresses allocated for private use are as follows:

• 10.0.0.0/8
• 172.16.0.0/12
• 192.168.0.0/16

Most enterprise networks use addresses from these blocks for their workstations. It doesn't matter if other organizations use the same addresses also, because the workstations are never directly connected to the same network.