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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.
Ranges There are two types of IP ranges CIDR ranges, e.g. 123.123.123.0/24; Non-CIDR ranges; The latter are frequently found in the allocation of IP-address ranges by for instance a provider to a customers or DHCP allocated addresses for certain purposes, as can be found by querying the WHOIS dataase of a RIR.
Each / 8 block contains 256 3 = 2 24 = 16,777,216 addresses, which covers the whole range of the last three delimited segments of an IP address. This means that 256 /8 address blocks fit into the entire IPv4 space.
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.
Since the introduction of CIDR, however, the assignment of an IP address to a network interface requires two parameters, the address and a subnet mask. Given an IPv4 source address, its associated subnet mask, and the destination address, a router can determine whether the destination is on a locally connected network or a remote network.
Reverse DNS queries are used to map IP addresses to domain names. They are PTR queries for subdomains of in-addr.arpa (for IPv4 addresses) [ 3 ] and ip6.arpa (for IPv6 addresses). [ 4 ] For example, to find the domain name associated with the IP address 203.0.113.22, one would send a PTR query for 22.133.0.203.in-addr.arpa .