Caution Do not confuse an IP subnet which is a logical division of a network with Ethernet segments which refer to physical divisions of an Ethernet network

Essentially, when you define a subnet mask, you add more bits to the default network mask for that address class. If you have a class B network, for example, the default network mask would be Then, if you decide to divide your network into 128 subnetworks, each of which has 512 hosts, you would designate 7 bits from the host address space as the subnet address. Thus, the subnet mask becomes

Using Supernets or CIDR

There are so few class A and B network addresses that they are becoming scarce. Class C addresses are more plentiful, but the proliferation of class C addresses has introduced a unique problem. Each class C address needs an entry in the network routing tables — the tables that contain information about how to locate any network on the Internet. Too many class C addresses means too many entries in the routing tables, which causes the router's performance to deteriorate. One way to get around this problem is to ignore the predefined address classes and let the network address be any number of bits. All you need is for the network mask to figure out which part of the 32-bit IP address is the network address. Based on this idea the Classless Inter-Domain Routing (CIDR)—docu-mented in RFC 1519 —was developed to enable routing of contiguous blocks of class C addresses with a single entry in the routing table. CIDR is used in the Internet as the primary mechanism to improve scalability of the Internet routing system.

The basis of CIDR is the idea of supernets —arbitrarily sized networks created by combining contiguous class C addresses that satisfy some criteria. For example, to create a supernet from two class C networks, the two network addresses must satisfy the following properties:

• The network addresses must be consecutive (for example, and are consecutive class C addresses).

• The third number of the first network address must be divisible by 2 (for example, the third number of is 18, and 18 is divisible by 2).

Thus, you could combine and into a single block, but you cannot combine with because 15 is not divisible by 2. When you create a supernet of two class C networks, the network can have up to 512 host addresses, and the network mask becomes, which leaves 9 bits for the host address.

You can also supernet any number of class C networks in powers of two. The only requirement is that the third number (in the dotted-decimal notation) of the first address must be divisible by the number of networks you are combining. Thus, if you are supernetting eight networks, the third number of the first address must be divisible by 8. Thus, you could supernet the following eight consecutive class C networks:

The network mask of this supernet would be, which provides for 21 bits of network address and leaves 11 bits for 8 x 256 = 2,048 host addresses.

Such a network address is written with the notation /21 to indicate that there are 21 bits in the network address.

Learning about IPv6

When the 4-byte IP address was created, the number of addresses seemed to be adequate. Now, however, class A and B addresses are running out, and class C addresses are being depleted at a fast rate. The Internet Engineering Task Force (IETF) recognized the potential for running out of IP addresses in 1991, and work began then on the next-generation IP addressing scheme, named IPng, which will eventually replace the old 4-byte addressing scheme (called IPv4, for IP Version 4).

Several alternative addressing schemes for IPng were proposed and debated. The final contender, with a 128-bit (16-byte) address, was dubbed IPv6 (for IP Version 6). Secret On September 18, 1995, the IETF declared the core set of IPv6 addressing protocols to be an IETF Proposed Standard. By now, there are many RFCs dealing with various aspects of IPv6, from IPv6 over PPP for the transmission of IPv6 packets over Ethernet.

IPv6 is designed to be an evolutionary step from IPv4. The proposed standard provides direct interoperability between hosts using the older IPv4 addresses and any new IPv6 hosts. The idea is that users can upgrade their systems to use IPv6 when they want and that network operators are free to upgrade their network hardware to use IPv6 without affecting current users of IPv4. Sample implementations of IPv6 are being developed for many operating systems, including Linux. For more information about IPv6 in Linux, consult the Linux IPv6 HOWTO at

The IPv6 128-bit addressing scheme allows for 170,141,183,460,469,232,000,000,000, 000,000,000,000 unique hosts! That should last us for a while!

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