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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 is examined, it will be seen that these network addresses have this common 23-bit prefix:

> 11001111 00010101 0011011

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:

> 207.21.0.0/16

When CIDR enabled ISPs to hierarchically distribute and manage blocks of contiguous addresses, IPv4 address space enjoyed the following benefits:

- Efficient allocation of addresses
- Reduced number of routing table entries