Classless IP Addressuing


The Move Towards Classless Addressing
By 1992, members of the IETF (Internet Engineering Task Force) had serious concerns about the exponential growth of the Internet and the limited scalability of Internet routing tables. They were also concerned with the eventual exhaustion of 32-bit IPv4 address space. The depletion of the class B address space was occurring so fast that within two years there would be no more class B addresses available (RFC 1519). This depletion was occurring because every organization that requested and obtained approval for IP address space received an entire classful network address - either a class B with 65,534 host addresses or a class C with 254 host addresses. One fundamental cause of this problem was the lack of flexibility. No class existed to serve a mid-sized organization that needed thousands of IP addresses but not 65,000.
In 1993, IETF introduced Classless Inter-Domain Routing, or CIDR (RFC 1517). CIDR allowed for:
More efficient use of IPv4 address space
Prefix aggregation, which reduced the size of routing tables
To CIDR-compliant routers, address class is meaningless. The network portion of the address is determined by the network subnet mask, also known as the network prefix, or prefix length (/8, /19, etc.). The network address is no longer determined by the class of the address.
ISPs could now more efficiently allocate address space using any prefix length, starting with /8 and larger (/8, /9, /10, etc.). ISPs were no longer limited to a /8, /16, or /24 subnet mask. Blocks of IP addresses could be assigned to a network based on the requirements of the customer, ranging from a few hosts to hundreds or thousands of hosts.

CIDR and Route Summarization:



CIDR uses Variable Length Subnet Masks (VLSM) to allocate IP addresses to subnets according to individual need rather than by class. This type of allocation allows the network/host boundary to occur at any bit in the address. Networks can be further divided or subnetted into smaller and smaller subnets.
Just as the Internet was growing at an exponential rate in the early 1990s, so were the size of routing tables that were maintained by Internet routers under classful IP addressing. CIDR allowed for prefix aggregation, which you already know as route summarization
"Static Routing" that you can create one static route for multiple networks. Internet routing tables were now able to benefit from the same type of aggregation of routes. The ability for routes to be summarized as a single route and helped reduce the size of Internet routing tables.
In the figure, notice that ISP1 has four customers, each with a variable amount of IP address space. However, all of the customer address space can be summarized into one advertisement to ISP2. The 192.168.0.0/20 summarized or aggregated route includes all the networks belonging to Customers A, B, C, and D. This type of route is known as a supernet route. A supernet summarizes multiple network addresses with a mask less than the classful mask.
Propagating VLSM and supernet routes requires a classless routing protocol, because the subnet mask can no longer be determined by the value of the first octet. The subnet mask now needs to be included with the network address. Classless routing protocols include the subnet mask with the network address in the routing update

Links:

"Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy," http://www.ietf.org/rfc/rfc1519
"Internet Protocol v4 Address Space," http://www.iana.org/assignments/ipv4-address-space





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