Ipv6 Colon-Separated-Hexadecimal Notation; Cidr Notation - Cisco ASR 5500 Installation Manual

Initial System Configuration

IPv6 Colon-Separated-Hexadecimal Notation

An Internet Protocol Version 6 (IPv6) address has two logical parts: a 64-bit network prefix and a 64-bit host
address part. An IPv6 address is represented by eight groups of 16-bit hexadecimal values separated by colons
(:).
A typical example of a full IPv6 address is 2001:0db8:85a3:0000:0000:8a2e:0370:7334.
The hexadecimal digits are case-insensitive.
The 128-bit IPv6 address can be abbreviated with the following rules:
• Leading zeroes within a 16-bit value may be omitted. For example, the address
fe80:0000:0000:0000:0202:b3ff:fe1e:8329 may be written as fe80:0:0:0:202:b3ff:fe1e:8329
• One group of consecutive zeroes within an address may be replaced by a double colon. For example,
fe80:0:0:0:202:b3ff:fe1e:8329 becomes fe80::202:b3ff:fe1e:8329.
IPv6 allows 128 bits for an Internet Protocol address and can support 2
(340,282,366,920,938,000,000,000,000,000,000,000,000) internet addresses.

CIDR Notation

Classless Inter-Domain Routing (CIDR) notation is a compact specification of an Internet Protocol address
and its associated routing prefix. It is used for both IPv4 and IPv6 addressing in networking architectures.
CIDR is a bitwise, prefix-based standard for the interpretation of IP addresses. It facilitates routing by allowing
blocks of addresses to be grouped into single routing table entries. These groups (CIDR blocks) share an initial
sequence of bits in the binary representation of their IP addresses.
CIDR notation is constructed from the IP address and the prefix size, the latter being the number of leading
1 bits of the routing prefix. The IP address is expressed according to the standards of IPv4 or IPv6. It is
followed by a separator character, the slash (/) character, and the prefix size expressed as a decimal number.
The address may denote a single, distinct, interface address or the beginning address of an entire network. In
the latter case the CIDR notation specifies the address block allocation of the network. The maximum size of
the network is given by the number of addresses that are possible with the remaining, least-significant bits
below the prefix. This is often called the host identifier.
For example:
• the address specification 192.168.100.1/24 represents the given IPv4 address and its associated routing
prefix 192.168.100.0, or equivalently, its subnet mask 255.255.255.0.
• the IPv4 block 192.168.0.0/22 represents the 1024 IPv4 addresses from 192.168.0.0 to 192.168.3.255.
• the IPv6 block 2001:DB8::/48 represents the IPv6 addresses from 2001:DB8:0:0:0:0:0:0 to
2001:DB8:0:FFFF:FFFF:FFFF:FFFF:FFFF.
• ::1/128 represents the IPv6 loopback address. Its prefix size is 128, the size of the address itself, indicating
that this facility consists of only this one address. An application sending a packet to this address will
get the packet back after it is looped back by the IPv6 stack. The equivalent IPv4 local host address is
127.0.0.1.
The number of addresses of a subnet defined by the mask or prefix can be calculated as 2
which the address size for IPv4 is 32 and for IPv6 is 128. For example, in IPv4, a mask of /29 gives: 2
3
2 = 8 addresses.
IP Address Notation
128
address size - mask
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