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2.2 | ![]() |
IP Addressing Crisis and Solutions | |
2.2.4 | ![]() |
Supernetting and address allocation |
Consider Company XYZ, which requires addresses for 400 hosts. Under
the classful addressing system, XYZ could apply to a central Internet
address authority for a Class B address. If the company got the Class
B address and then used it to address one logical group of 400 hosts,
tens of thousands of addresses would be wasted. A second option for
XYZ would be to request two Class C network numbers, yielding 508, or
2 * 254, host addresses. The drawback to this approach is that XYZ
would have to route between its own logical networks. Also, Internet
routers would still need to maintain two routing table entries for the
XYZ network, rather than just one. Under a classless addressing system, supernetting allows XYZ to get the address space that it needs without wasting addresses or increasing the size of routing tables unnecessarily. Using CIDR, XYZ asks for an address block from its Internet Service Provider, not a central authority such as the InterNIC. The ISP assesses the needs of XYZ and allocates address space from its own large CIDR block of addresses. Providers assume the burden of managing address space in a classless system. With this system, Internet routers keep only one summary route, or supernet route, to the provider network. The provider keeps routes that are more specific to its customer networks. This method drastically reduces the size of Internet routing tables. In the following example, XYZ receives two contiguous Class C
addresses, 207.21.54.0 and 207.21.55.0. If the shaded portion of
Figure
When supernetted with a 23-bit mask, 207.21.54.0/23, the address
space provides well over 400, or 29, host addresses
without the tremendous waste of a Class B address. With the ISP acting as the
addressing authority for a CIDR block of addresses, the ISP's customer
networks, which include XYZ, can be advertised among Internet routers
as a single supernet. The ISP manages a block of 256 Class C addresses
and advertises them to the world using a 16-bit prefix:
When CIDR enabled ISPs to hierarchically distribute and manage blocks of contiguous addresses, IPv4 address space enjoyed the following benefits:
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