Subnetting assignment

Article is provided courtesy of Cisco Press. Article Description This quick and dirty look at the notation differences between Subnetting assignment and IPv6 will ensure that you know what you need to to run IPv6 effectively on your network. With the standard decimal notated IPv4 address and network. IPv6 address and network, it can get confusing.

Sean Wilkins explains two of the most commonly used IPv6 ranges and applications, and how they can be used to calculate IPv6 addressing ranges. While the general concepts behind the subnetting of IPv6 are not all that different from IPv4 subnetting, it can be a little challenging to get around the size and notation differences between a standard decimal notated IPv4 address and network and the standard hexadecimal notated IPv6 address and network. IPv6 Addressing Types and Assignment Structures Before going forward that far it is important to note that like IPv4, the placement of the devices that will be allocated with IPv6 addresses can affect the numbers and the method of assignment. RFC 1918 IPv4 private address ranges.

The assignment of Unique Local IPv6 addresses follows a structure as shown in Figure 1. The assignment of Global Unicast addresses follows a structure as shown in Figure 2. Whereas classful network design for IPv4 sized the network prefix as one or more 8-bit groups, resulting in the blocks of Class A, B, or C addresses, Classless Inter-Domain Routing allocates address space to Internet service providers and end users on any address bit boundary. CIDR introduced a new method of representation for IP addresses, now commonly known as CIDR notation, in which an address or routing prefix is written with a suffix indicating the number of bits of the prefix, such as 192. CIDR introduced an administrative process of allocating address blocks to organizations based on their actual and short-term projected needs. The aggregation of multiple contiguous prefixes resulted in supernets in the larger Internet, which whenever possible are advertised as aggregates, thus reducing the number of entries in the global routing table. An IP address is interpreted as composed of two parts: a network-identifying prefix followed by a host identifier within that network.

In the previous classful network architecture, IP address allocations were based on the bit boundaries of the four octets of an IP address. This led to inefficiencies in address use as well as inefficiencies in routing, because it required a large number of allocated class-C networks with individual route announcements, being geographically dispersed with little opportunity for route aggregation. IP address space and the routing of IP packets was not scalable. CIDR notation is a compact representation of an IP address and its associated routing prefix. The number is the count of leading 1 bits in the subnet mask. Larger values here indicate smaller networks.

The IP address is expressed according to the standards of IPv4 or IPv6. The address may denote a single, distinct interface address or the beginning address of an entire network. The aggregation of these bits is often called the host identifier. Its prefix length is 128 which is the number of bits in the address. For IPv4, CIDR notation is an alternative to the older system of representing networks by their starting address and the subnet mask, both written in dot-decimal notation. 128 for IPv6 and 32 for IPv4.

A subnet mask is a bitmask that encodes the prefix length associated with an IPv4 address or network in quad-dotted notation: 32 bits, starting with a number of 1 bits equal to the prefix length, ending with 0 bits, and encoded in four-part dotted-decimal format: 255. A subnet mask encodes the same information as a prefix length, but predates the advent of CIDR. In CIDR notation, the prefix bits are always contiguous. Given this constraint, a subnet mask and CIDR notation serve exactly the same function. CIDR is principally a bitwise, prefix-based standard for the representation of IP addresses and their routing properties. It facilitates routing by allowing blocks of addresses to be grouped into single routing table entries.

These groups, commonly called CIDR blocks, share an initial sequence of bits in the binary representation of their IP addresses. The dotted decimal portion is the IPv4 address. The number following the slash is the prefix length, the number of shared initial bits, counting from the most-significant bit of the address. When emphasizing only the size of a network, the address portion of the notation is usually omitted.