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Multiple IPv6 addresses can be entered, as shown in the following example which uses a long command, not shown for brevity. The first two rows show that 3 of the entered addresses are in a /33 range, while 72 are in a /64. Blocking the /33 would affect 2G /64, that is, over 2 billion /64 allocations, but would block only 3 of the given addresses.
Some large / 8 blocks of IPv4 addresses, the former Class A network blocks, are assigned in whole to single organizations or related groups of organizations, either by the Internet Corporation for Assigned Names and Numbers (ICANN), through the Internet Assigned Numbers Authority (IANA), or a regional Internet registry.
Used for link-local addresses [5] between two hosts on a single link when no IP address is otherwise specified, such as would have normally been retrieved from a DHCP server 172.16.0.0/12 172.16.0.0–172.31.255.255
Shorter CIDR prefixes match more addresses, while longer prefixes match fewer. In the case of overlaid CIDR blocks, an address can match multiple CIDR prefixes of different lengths. CIDR is also used for IPv6 addresses and the syntax semantic is identical. The prefix length can range from 0 to 128, due to the larger number of bits in the address.
If an ISP deploys a CGN and uses private Internet address space [2] (networks 10.0.0.0 / 8, 172.16.0.0 / 12, 192.168.0.0 / 16) to connect their customers, there is a risk that customer equipment using an internal network in the same range will stop working.
In support of link-local multicasts which do not use IGMP, any IPv4 multicast address that falls within the *.0.0.0 / 24 and *.128.0.0 / 24 ranges will be broadcast to all ports on many Ethernet switches, even if IGMP snooping is enabled, so addresses within these ranges should be avoided on Ethernet networks where the functionality of IGMP ...
2 IGMP Internet Group Management Protocol: RFC 1112: 0x03 3 GGP Gateway-to-Gateway Protocol: RFC 823: 0x04 4 IP-in-IP IP in IP (encapsulation) RFC 2003: 0x05 5 ST Internet Stream Protocol: RFC 1190, RFC 1819: 0x06 6 TCP Transmission Control Protocol: RFC 793: 0x07 7 CBT Core-based trees: RFC 2189: 0x08 8 EGP Exterior Gateway Protocol: RFC 888 ...
IP addresses in dot-decimal notation are also presented in CIDR notation, in which the IP address is suffixed with a slash and a number, used to specify the length of the associated routing prefix. For example, 127.0.0.1/8 specifies that the IP address has an eight-bit routing prefix, and therefore the subnet mask 255.0.0.0.