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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.
For example, the subnet mask for a routing prefix that is composed of the most-significant 24 bits of an IPv4 address is written as 255.255.255.0. The modern standard form of specification of the network prefix is CIDR notation, used for both IPv4 and IPv6.
Since it would not be practical to manually provision networks at scale, in IPv6 networking, DHCPv6 prefix delegation (RFC 3633; RFC 8415 § 6.3) is used to assign a network address prefix and automate configuration and provisioning of the public routable addresses for the network. In the typical case of a home network, for example, the home ...
The term subnet mask is only used within IPv4. Both IP versions however use the CIDR concept and notation. In this, the IP address is followed by a slash and the number (in decimal) of bits used for the network part, also called the routing prefix. For example, an IPv4 address and its subnet mask may be 192.0.2.1 and 255.255.255.0, respectively.
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.
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: 255.255.255.0. A subnet mask encodes the same information as a prefix length but ...
The large addressing space of IPv6 can still be defeated depending on the actual prefix length given by the carrier. It is not uncommon to be handed a /64 prefix – the smallest recommended subnet – for an entire home network, requiring a variety of techniques to be used to manually subdivide the range for all devices to remain reachable. [29]
In this case, the longest prefix of the candidate routes is 192.168.20.16/28, since its subnet mask (/28) is longer than the other entry's mask (/16), making the route more specific. Forwarding tables often contain a default route, which has the shortest possible prefix match, to fall back on in case matches with all other entries fail.