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Supernetting and Classless Interdomain Routing

With the recent growth of the Internet, it became clear to the Internet authorities that the class B network IDs would soon be depleted. For most organizations, a class C network ID does not contain enough host IDs and a class B network ID has enough bits to provide a flexible subnetting scheme within the organization.

The Internet authorities devised a new method of assigning network IDs to prevent the depletion of class B network IDs. Rather than assigning a class B network ID, the Internet Network Information Center (InterNIC) assigns a range of class C network IDs that contain enough network and host IDs for the organization's needs. This is known as supernetting. For example, rather than allocating a class B network ID to an organization that has up to 2,000 hosts, the InterNIC allocates a range of 8 class C network IDs. Each class C network ID accommodates 254 hosts, for a total of 2,032 host IDs.

While this technique helps conserve class B network IDs, it creates a new problem. Using conventional routing techniques, the routers on the Internet now must have8class C network ID entries in their routing tables to route IP packets to the organization. To prevent Internet routers from becoming overwhelmed with routes, atechnique called Classless Interdomain Routing (CIDR) is used to collapse multiple network ID entries into a single entry corresponding to all of the class C network IDs allocated to that organization.

Conceptually, CIDR creates the routing table entry: {Starting Network ID, count}, where Starting Network ID is the first class C network ID and the count is the number of class C network IDs allocated. In practice, a supernetted subnet mask is used to convey the same information. To express the situation where 8 class C network IDs are allocated starting with Network ID 220.78.168.0:

Starting Network ID
220.78.168.0 = 100111100 01001110 10101000 00000000

Ending Network ID
220.78.175.0 = 100111100 01001110 10101111 00000000

Note that the first 21 bits (underlined) of all the above Class C network IDs are the same. The last three bits of the third octet vary from 000 to 111. The CIDR entry in the routing tables of the Internet routers becomes:

Network ID Subnet Mask Subnet Mask (binary)

220.78.168.0 255.255.248.0 1111111111 11111111 11111000 00000000

In network prefix notation, the CIDR entry is 220.78.168.0/21.

A block of addresses using CIDR is known as a CIDR block.

Note Since subnet masks are used to express the count, class-based network IDs must be allocated in groups corresponding to powers of two.

In order to support CIDR, routers must be able to exchange routing information in the form of {Network ID, Subnet Mask} pairs. RIP for IP version 2, OSPF, and BGPv4 are routing protocols that support CIDR. RIP for IP version 1 does not support CIDR.

The Address Space Perspective

The use of CIDR to allocate addresses promotes a new perspective on IP network IDs. In the above example, the CIDR block {220.78.168.0, 255.255.248.0} can be thought of in two ways:

• A block of 8 class C network IDs.
• An address space in which 21 bits are fixed and 11 bits are assignable.

In the latter perspective, IP network IDs lose their class-based heritage and become separate IP address spaces, subsets of the original IP address space defined by the 32-bit IP address. Each IP network ID (class-based, subnetted, CIDR block), is an address space in which certain bits are fixed (the network ID bits) and certain bits are variable (the host bits). The host bits are assignable as host IDs or, using subnetting techniques, can be used in whatever manner best suits the needs of the organization.

Public and Private Addresses

If your intranet is not connected to the Internet, any IP addressing can be deployed. If direct (routed) or indirect (proxy or translator) connectivity to the Internet is desired, then there are two types of addresses employed on the Internet, public addresses and private addresses.

Public Addresses

Public addresses are assigned by InterNIC and consist of class-based network IDs or blocks of CIDR-based addresses (called CIDR blocks) that are guaranteed to be globally unique to the Internet.

When the public addresses are assigned, routes are programmed into the routers of the Internet so that traffic to the assigned public addresses can reach their locations. Traffic to destination public addresses are reachable on the Internet.

For example, when an organization is assigned a CIDR block in the form of a network ID and subnet mask, that {network ID, subnet mask} pair also exists as a route in the routers of the Internet. IP packets destined to an address within the CIDR block are routed to the proper destination.

Illegal Addresses

Private intranets that have no intent on connecting to the Internet can choose any addresses they want, even public addresses that have been assigned by the InterNIC. If an organization later decides to connect to the Internet, its current address scheme may include addresses already assigned by the InterNIC to other organizations. These addresses would be duplicate or conflicting addresses and are known as illegal addresses. Connectivity from illegal addresses to Internet locations is not possible.

For example, a private organization chooses to use 207.46.130.0/24 as its intranet address space. The public address space 207.46.130.0/24 has been assigned to the Microsoft corporation and routes exist on the Internet routers to route all packets destined to IP addresses on 207.46.130.0/24 to Microsoft routers. As long as the private organization does not connect to the Internet, there is no problem, since the two address spaces are on separate IP internetworks. If the private organization then connected directly to the Internet and continued to use 207.46.130.0/24 as its address space, then any Internet response traffic to locations on the 207.46.130.0/24 network would be routed to Microsoft routers, not to the routers of the private organization.

Private Addresses

Each IP node requires an IP address that is globally unique to the IP internetwork. In the case of the Internet, each IP node on a network connected to the Internet requires an IP address that is globally unique to the Internet. As the Internet grew, organizations connecting to the Internet required a public address for each node on their intranets. This requirement placed a huge demand on the pool of available public addresses.

When analyzing the addressing needs of organizations, the designers of the Internet noted that for many organizations, most of the hosts on the organization's intranet did not require direct connectivity to Internet hosts. Those hosts that did require a specific set of Internet services, such as the World Wide Web access and e-mail, typically access the Internet services through application layer gateways such as proxy servers and e-mail servers. The result is that most organizations only required a small amount of public addresses for those nodes (such as proxies, routers, firewalls, and translators) that were directly connected to the Internet.

For the hosts within the organization that do not require direct access to the Internet, IP addresses that do not duplicate already-assigned public addresses are required. To solve this addressing problem, the Internet designers reserved a portion of the IP address space and named this space the private address space. An IP address in the private address space is never assigned as a public address. IP addresses within the private address space are known as private addresses. Because the public and private address spaces do not overlap, private addresses never duplicate public addresses.

The private address space specified in RFC 1597 is defined by the following three address blocks: